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Blood, 1 April 2006, Vol. 107, No. 7, pp. 2959-2967. Prepublished online as a Blood First Edition Paper on December 15, 2005; DOI 10.1182/blood-2005-07-2670. This Article Abstract Full Text (PDF) All Versions of this Article: 2005-07-2670v1 107/7/2959    most recent Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Rights and Permissions Citing Articles Citing Articles via HighWire Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Priestley, G. V. Articles by Papayannopoulou, T. Search for Related Content PubMed PubMed Citation Articles by Priestley, G. V. Articles by Papayannopoulou, T. Related Collections Cell Adhesion and Motility Stem Cells in Hematology Hematopoiesis and Stem Cells Social Bookmarking                   What's this?  Previous Article  |  Table of Contents  |  Next Article  STEM CELLS IN HEMATOLOGY Lack of 4 integrin expression in stem cells restricts competitive function and self-renewal activity Gregory V. Priestley, Linda M. Scott, Tatiana Ulyanova, and Thalia Papayannopoulou From the Division of Hematology, University of Washington, Seattle, WA; and the University Department of Haematology, Cambridge Institute for Medical Research, Cambridge, United Kingdom.    Abstract Top Abstract Introduction Materials and methods Results Discussion References   Alpha4 integrin or VLA4 (CD49d/CD29) is a multitask molecule with wide expression within and outside the hematopoietic system. Because targeted ablation of 4 integrin leads to embryonic lethality, to study its effects on adult hematopoiesis, we used animals with conditional excision of 4 integrin (4/) in hematopoietic cells. In such animals, we previously documented weakened bone marrow retention of progenitor cells during homeostasis and impaired homing and short-term engraftment after transplantation. In the present study we show that long-term repopulating cells lacking 4 integrins display a competitive disadvantage in hematopoietic reconstitution compared to normal competitors. Although initial dominance of 4+ competitors is due to their better homing and proliferative expansion early after transplantation, a progressive decline in contribution of 4/ hematopoiesis is compatible with neither normal homing nor normal function of 4/ hematopoietic stem cells (HSCs) in post-homing hematopoiesis. In the absence of 4+ competitor cells, 4/ HSCs can establish long-term hematopoiesis in primary recipients, however, some resurgence of host hematopoiesis is evident, and it becomes dominant in secondary transplants, so that no survivors with exclusively 4/ cells are seen in tertiary transplants. Collectively, our data provide compelling evidence that under regenerative stress 4 integrin assumes a greater importance than for maintenance of steady-state hematopoiesis.    Introduction Top Abstract Introduction Materials and methods Results Discussion References   The 4 integrin (CD49d), or VLA4, the heterodimer of 4 and 1 integrin (CD49d/CD29), is widely expressed within and outside the hematopoietic system and exerts important functional control over many physiologic, as well as pathologic or neoplastic, processes.1-6 Among hematopoietic cells, 4 integrin is expressed in a constitutively active stage in primitive cells and in an inactive state in several classes of mature cells.7 It exercises a decisive influence on the migration and recruitment patterns of mature cells, especially lymphocytes, to several tissues,8 whereas within bone marrow (BM), it plays a critical role in the interactions of hematopoietic cells with microenvironmental cells and their matrix.9 Because of its ability to not only serve as adhesion receptor, but also to execute bi-directional signaling (outside-in and inside-out10) and to interact with cytokines/chemokines and other integrins expressed by hematopoietic cells or/and microenvironmental cells, 4 integrin is uniquely poised to influence interactions of hematopoietic cells with their environment. Early studies of 4 integrin on hematopoiesis relied on the use of antifunctional antibodies, both in vitro9 and in vivo.11 Targeted ablation of 4 integrin caused embryonic lethality unrelated to hematopoietic effects,12 but studies in chimeric mice showed modest effects on fetal liver hematopoiesis and no contribution to adult hematopoiesis beyond the first month of postnatal life.13,14 To study the effects of 4 integrin on adult hematopoiesis, we engineered conditional knockout mice in which the 4 alleles can be disrupted upon treatment of adult animals with interferon or its inducer poly(I)-poly(C).15 Adult animals with conditional excision of 4 integrin maintain a quantitatively normal hematopoiesis at homeostasis but display alterations in biodistribution of progenitor cells with sustained elevations in circulation and in the spleen. Such an effect is consequent to their compromised retention within the BM in the absence of 4 integrin, an effect reminiscent of the mobilization seen in vivo with anti-4 antibodies.11 A counterpart of this effect is a partial impairment in BM homing and in short-term engraftment.15 To test the ability of 4-deficient cells for long-term, durable engraftment in irradiated recipients and the self-renewal properties of 4/ hematopoietic stem cells (HSCs), in the present study we carried out a series of transplantation experiments with or without competitor cells. Our results suggest that absence of 4 integrin in hematopoietic cells greatly compromises their competitive ability for hematopoietic reconstitution and their self-renewal capacity in serial transplantations.    Materials and methods Top Abstract Introduction Materials and methods Results Discussion References   Mice Mx.cre+ 4f/f and Mx.cre+ 4/ mice were generated in our laboratory as described previously.15 Three injections of poly(I)-poly(C) every other day were used for ablation of adult animals, whereas in neonates, 2 injections were used during the first week of postnatal life. Alpha4 ablation was tested 2 weeks later. Ablated animals are referred to as 4/, and the animals with more than 5% 4+ cells in their peripheral blood were not included in experiments. Mice were bred and maintained in the specific pathogen-free facility at the University of Washington in accordance with the Institutional Animal Care Use Committee guidelines. Antibodies, cell staining, and FACS analysis Cells from mouse tissues (bone marrow, peripheral blood, or spleen) were stained using the following directly conjugated antibodies from BD Biosciences (San Diego, CA): B220 (RA3-682), CD3 (17A2), CD4 (RM4-5), CD8 (53-6.7), CD45 (30F-11), TER119, Mac-1 (M1/70), Gr-1 (RB6-8C5), CD117 (c-kit, 2B8), Sca-1 (D7), CD29 (1, Ha2/5), anti-CD48 (HM48-1), CD49e (5, 5H10-27 [MFR5]), anti-CD150, and anti-47 (DATK32), directly conjugated CD150 (SLAM, IPO-3) purchased from eBioscience (San Diego, CA) and directly conjugated CD49d (anti-4, PS/2) antibody was purchased from Southern Biotechnology Associates, Birmingham, AL. The cocktail of antibodies used for staining of lineage-committed cells included CD3 (17A2), Mac-1, TER119, and Gr-1, B220. A FACSCalibur (BD Biosciences) and CELLQuest software were used for cell analysis. Proliferation assessment Assessment of proliferative status of BM cells of 4/ and of controls (4f/f) was done using a single injection of BrDU (250 mg/kg body weight) and analyzing the cells 3 hours later, as described.16 BrDU staining was carried out according to the manufacturer's instructions and was combined with anti-CD45-PE or anti-kit-APC. Competitive repopulation Pooled BM cells from 4/ mice (= test cells) were mixed in different proportions (250 000-750 000) with 4f/f/cre- cells (250 000) (= competitor cells) and transplanted by intravenous injection (0.5-1.0 x 106) into lethally irradiated (1200 cGy at a dose rate of > 100 cGy/min) wild-type (WT) recipients using a cesium source. In addition to already ablated 4/, 4f/fcre+ BM cells before ablation were competed (1:1) with 4+/+ Mx.cre+ cells. Recipients of the latter pool of BM were ablated (by poly(I)-poly(C)) 4 weeks after transplantation. BM and peripheral blood (PB) cells of all recipients were evaluated at several points after transplantation by fluorescence-activated cell sorting (FACS), colony-forming unit-in culture (CFU-C) in the presence or absence of G418, and by genomic analysis (polymerase chain reaction [PCR]). At chosen times after transplantation, selected primary recipients served as donors for secondary transplants, which were similarly analyzed 3 to 6 months later. Transplantation of 4-deficient BM or PB Bone marrow cells (1-5 x 106 cells/recipient) from 4 ablated mice were infused into lethally irradiated recipients (1150 cGy whole body irradiation with a 137Cs source). Both splenectomized and nonsplenectomized animals were used as recipients. Recipients were analyzed 2, 10, 16, and 56 weeks later, and serial transplantations (secondary and tertiary) were carried out from selected primary and secondary recipient (4+/+ or 4/) animals. Evaluation of hematopoietic reconstitution was carried out by FACS for different cell lineages, by hematopoietic-cell quantitation in BM, PB, and spleen, and through CFU-C assays with or without G418. Presence of "floxed" (f), "deleted" (), or "WT" alleles were used in genomic analysis to assess donor-cell contribution. Peripheral blood also was used for transplantation (0.3 mL from 4/ or 4+/+ animals). Recipients were analyzed 4 months later, using the approaches described earlier in "Materials and methods." All procedures were approved by the University of Washington Institutional Animal Care and Use Committee (IACUC). Clonogenic progenitor assays Colony-forming-unit-in culture (CFU-C) assays were performed using a methylcellulose mixture (Methocult GF, Stem Cell Technologies, Vancouver, BC, Canada), as described previously.15 BM or PB cells were inoculated for culture, and colonies were identified morphologically in culture plates as erythroid (BFUe) or granulocyte/macrophage (CFU-GM) or CFU-Mixed. Whenever appropriate, G418 was added to cultures to test for the presence of the neo-allele in 4-unexcised cells (4f/f), providing complementary evidence to that from genomic PCR analysis. Genomic PCR. Genomic DNA was isolated using a Gentra Kit (Gentra Systems, Minneapolis, MN). The following primer combinations were used for PCR: 4 gene-specific primers: F1 (5'-CCACCTGGTGTATGAAAGC-3'), F2 (5'-CGGGATCAGAAAGAATCCAAA-3'), and R1 (5'-CTGGCATGGGGTTAAAATTG-3'); this primer combination allows discrimination between WT, deleted (), and floxed (f) alleles; neo-specific primers: sense (5'-GCACGCAGGTTCTCCGGC-3'), antisense (5'-GTCCTGATAGCGGTCCGCC-3'); cre-specific primers; sense (5'-CATTTGGGCCAGCTAAACAT-3'), antisense (5'-TAAGCAATCCCCAGAAATGC-3'). Statistical analysis. Data shown are means ± standard error of the mean (SEM). Statistical analyses were performed by using a Student t test.    Results Top Abstract Introduction Materials and methods Results Discussion References   Normal immunophenotypic profile of 4/ stem cells Previous studies in mice, in which excisions of 4 integrin in hematopoietic cells occurred early or late in their postnatal life, have shown normal quantitative hematopoiesis and normal lineage distribution at homeostasis. (Proportions of kit+ cells in BM, or of Gr-1+, Mac-1+, CD3+, and TER119+ were similar to those of normal cre(-) mice15). A modest deviation in the frequency of B cells (B220+) between BM and PB was seen, as well as significant and sustained augmentations in circulating progenitor (CFU-C) cells of all classes,15 as documented by clonal growth in vitro. To test whether the immunophenotypic profile of 4/ HSCs several months after 4-integrin excision is also quantitatively preserved, we assessed the proportion in BM of Lin-/Sca-1+/kit- (LSK) cells that contain an enriched population of HSCs. As seen in Figure 1A, we found no differences in the proportions of either Lin-/kit+ or Lin-/Sca-1+/kit+ (LSK) cells between 4/ and controls. More importantly, these populations were virtually negative for 4 expression (Figure 1B). Recently, an improved immunophenotype was described for stem cells (CD150+/CD48-), and we tested proportions of these cells between WT and 4/ mice. No quantitative differences between controls and 4/ BM cells were evident (CD150+/CD48+: 4.09% in controls and 6.6% in 4/; CD150+/CD48- were 0.014% and 0.018%, respectively, and all CD150+CD48+ cells were 4 negative (Figure 1C). In addition to the immunophenotypic profile, we tested the proliferative status of kit+ cells in BM of +/+ animals (n = 4) and of 4/ (n = 8, before and after splenectomy). No significant differences in BrDU labeling were seen among the groups (BrDU+: 24.2% ± 3.6% vs 29.7% ± 2.0% among CD45+ cells, respectively, P = .24). Competitive repopulation with 4+/+ and 4/ cells To test whether the phenotypically defined 4/ HSCs are endowed with a normal functional capacity for long-term reconstitution of lethally irradiated recipients, we performed competitive repopulation experiments.17 Initially, we mixed equal numbers of BM nucleated cells from 4/ mice (ablated several months previously) and from BM of cre-4f/f mice (1:1 test to competitor ratio). Equal mixing was verified by immunophenotyping of pool populations and proportions of kit+ cells (Figure 2A). This 1:1 donor pool of cells was injected (0.5 + 0.5 x 106 cells/recipient) into a cohort of 10 lethally irradiated WT recipients. In this setting, hematopoiesis by 4/, 4f/f, or 4 WT cells can be genetically distinguished. To assess long-term donor contributions to hematopoiesis, PB from recipient mice was sampled at 16, 30, and 42 weeks after transplantation. Blood was analyzed by FACS (proportion of 4+ cells) and for progenitor content in the presence or absence of G418 for scoring neo-resistant (4f/f derived) or neo-sensitive (4/ or WT-derived) colonies. The proportion of 4+ cells (among CD45+ cells) at all times tested was between 65% and 75%. Proportions of Gr-1+ cells reflecting recent contributions by BM progenitors also were similar. One rather moribund animal was killed earlier at 12 weeks and showed 52.68% 4+ in PB (the expected contribution); however, in the BM he had 71.65% 4+ cells. All the kit+ in the BM were 4+, as was the great majority of Gr-1+, Mac-1+, TER119+, and B220+ cells (data not shown). This suggested that PB data may not accurately reflect BM hematopoiesis and that we may have an ongoing contribution by 4(+) at the expense of 4(-) cells. This concept was tested at 30 weeks by humanely killing 4 mice. All mice showed further increase in PB contribution by 4+ cells, but this was not statistically significant from earlier times. However, BM analysis from all 4 mice disclosed 97.45% ± 0.1% 4(+) cells compared to 77.49% ± 4.2% in PB (P = .02), suggesting complete replacement by 4(+) hematopoiesis. All CFU-C in vitro were G418 resistant, suggesting contribution by 4f/f competitor cells rather than host stem-cell resurgence. Finally, at 42 weeks after transplantation, 5 more mice were euthanized for analysis. Their PB cells were 73.14% ± 11.8% 4+, but the BM had 96.95% ± 1.65% 4+ cells. Again, a similar picture emerged to the one seen earlier at 30 weeks. These data suggested not only an inferior competition by 4/ cells starting early after transplantation, but also a progressive replacement of hematopoiesis by 4(+) cells long after hematopoietic reconstitution is stably established. They also validate the 4f/f competitor cells as functionally intact. To uncover the presence of any silent 4/ HSCs that had not contributed to hematopoiesis in the primary recipients, at 30 and 42 weeks after transplantation, 1 x 106 pooled BM cells from 4 mice (97.4% 4+) were given to each of 14 recipients, and BM pooled cells from 2 mice (90% 4[+]) were given to each of 10 recipients. The first cohort of recipients was tested at 27 weeks (11 mice) and the second at 13 weeks after (8 mice). All lineages within BM from both sets of animals showed 4+ hematopoiesis (> 98%) in BM, PB, and spleen. View larger version (43K): [in this window] [in a new window]   Figure 1.. FACS analysis of BM cells from 4-deficient or control mice. (A) Note the absence of 4 expression and the left shift in 1 integrin expression of BM cells from 4/ mice compared to controls (4 left panels). Solid lines represent isotype control antibody. (ii) Proportions of Lineage-/Sca-1+/kit+ (LSK) cells containing an enriched population of stem cells. Gated Lineage-/kit+ cells (top left) were tested for Sca-1 positivity (ii). No differences are seen between the 4/ (neonatally ablated and tested at 14 weeks) and control mice (14 weeks old), suggesting normal representation of LSK cells in 4-deficient mice. Furthermore, kit+ cells did not express integrin (panel B). In panel C, BM cells were labeled with anti-CD150 PE and anti-CD48 FITC. Among more than 2 x 105 cells analyzed, the proportion of CD150+ CD48- cells were 0.014% and 0.02% (2 experiments) in control and 0.18% and 0.03% in neonatally ablated 4/ animals of the same age (4-5 months). No 4+ cells were seen among CD150+CD48- cells from 4/ BM (data not shown). The proportion of CD150+CD48+ was 4.09% in controls and 6.62% in 4/. It is of interest that all kit+ cells were CD48+ in both sets of animals.   View larger version (44K): [in this window] [in a new window]   Figure 2.. Competitive repopulation with pooled 4+/+ (competitor cells) and 4/ (test cells) BM cells. (A) Proportion of 4+ cells in the pooled BM sample used for transplantation is depicted in the left panel, and proportion of kit+/4+ cells is shown in the right panel. (B) Blood samples from recipients who received transplants were assessed 16 to 42 weeks after transplantation for the presence of 4+ () or 4- () cells. At all times tested, the proportion of 4+ cells was 2:1 in PB (compared to input ratio of 1:1). (C) Comparison of PB and BM positivity (right panels) for 4 integrin in mice that received transplants and were killed at 30 (n = 4) or 42 (n = 5) weeks after transplantation (PB, ; BM from the same group of mice, ). Reconstitution by 4+ cells was higher in BM compared to concurrently tested PB cells (total cells or kit+ cells, left panels), suggesting ongoing replacement of hematopoiesis by 4+ competitor cells. Error bars represent standard error of the mean.   To further investigate the kinetics of the apparent competitive disadvantage of 4/ cells and to overcome a putative partial homing defect for 4/ HSC (long-term repopulating cells [LTRCs]), similar to the one documented for progenitor cells (CFU-C),15 we repeated the competitive repopulation experiments. Keeping the number of competitor cells constant, we used 1:1 and 3:1 (6:1 by CFU-C ratio) of test 4/ versus 4+/+ competitor cells (Figure 3A). These cells were transplanted into splenectomized recipients to avoid contributions of 4/ hematopoiesis by the spleen, which avidly sequesters 4/ cells. Furthermore, we wanted to test whether contributions by the spleen could account for the differences between PB and BM seen previously, instead of evoking the postulate of progressive decline of 4/ hematopoiesis. By 6 weeks, 4+ blood cells were above 60% in all recipients receiving either 1:1 or 3:1 donor cells (74.76% ± 2.4% for 1:1 and 60.65% ± 3.6% for 3:1). By 24 weeks (168 days) 4+ cells were 79% ± 7.0% for the 1:1 and 80.01% ± 4.9% for the 3:1 donor cohort. Thus, in accord with our earlier data, there was a progressive decline in the contribution by 4/ cells from 6 to 36 weeks (P = .04 for the 3:1 group of recipients) (Figure 3B). When mice were killed (36 weeks), 92.55% ± 11.5% of CFU-Cs from the BM of all recipients of 1:1 ratio were G418 resistant (ie, 4f/f derived), whereas in PB were 83.6% ± 18.4%, and in the recipients of 3:1 donor pool, they were 95.5% ± 2.4% and 80.84% ± 6.0%, respectively. Moreover, DNA analysis of BM cells from all these recipients (Figure 3C) confirmed the derivation of the great majority of multilineage hematopoiesis by 4f/f competitor cells (Figure 4). View larger version (41K): [in this window] [in a new window]   Figure 3.. Competitive repopulation experiments with 1:1 or 3:1 4/ (= test) to 4+/+ (= competitive) cell ratio. (A) FACS profiles of pooled 1:1 BM or 3:1 BM samples showing proportions of 4- () versus 4+ () cells (CFU-C ratio of T:C cells was 6:1 in the latter). (B) PB-cell evaluation for 4 positivity shows an early and progressive reconstitution by 4+ competitor cells from 6 to 36 weeks after transplantation (P = .04, , percent 4-; , percent 4+). (C) Genomic PCR in 14 surviving mice (7 from 1:1 and 7 from 3:1) from competitive repopulation at 36 weeks. Note the predominant presence of floxed (f) allele (= competitor) compared to deleted () one and a minor presence of WT allele from hosts (likely contaminating CD45- cells). Predominant reconstitution by 4f/f (= competitors) was also corroborated by CFU-C cultures, showing mostly G418-resistant colonies (4f/f are neo+) from competitor cells. Only 2 mice (no. 1 from 1:1 and no. 2 from 3:1) show appreciable numbers of () allele corresponding to higher proportions of 4- cells. Error bars represent standard error of the mean.   View larger version (20K): [in this window] [in a new window]   Figure 4.. Multilineage reconstitution by 4+ competitor cells. Expression of 4+ cells () among cells from different lineages in (A) BM, (B) lymph nodes (LN), and (C) thymus of mice transplanted with 3:1 (test:competitive) cell ratio and tested at 36 weeks after transplantation. ( show the proportion of 4- cells.) Proportions of 4+ cells () in all tested tissues after transplantation show only minor deviations from data in normal, nontransplanted tissues (ie, normal BM has 95%-99% 4+ cells), suggesting multilineage engraftment.   These data secured the fact that 4/ cells are out-competed by 4+ cells early during hematopoietic reconstitution and that any remaining contribution by 4/ cells is dwindling thereafter. Since the competitive advantage of 4+/+ cells was shown for the first few weeks after transplantation, the most likely interpretation is the superior homing of short-term engrafting 4+/+ cells. To eliminate homing differences in competitive repopulation, we decided to transplant pools composed of equal numbers of 4+/+/cre+ BM cells and 4f/f/cre+ cells that had not been ablated. This pool (1:1 ratio) consisted of 96.5% 4+ cells among kit+ at the time of transplantation. Upon hematopoietic reconstitution at approximately 4 weeks, we planned to induce 4 ablation in recipient animals (by poly(I)-poly(C) injections). Thus, at 4 weeks, we tested the blood of recipient animals and, to our surprise, instead of the expected approximately 95% 4 positivity, we had only 75.4% ± 1.9% in the entire group of 10 recipients. A proportion of 25% 4-negative cells in PB can be explained only by a partial 4 gene ablation occurring at posttransplantation period, presumably because of increased levels of endogenous interferon. Nevertheless, we ablated the animals, hoping to uncover additional contribution by 4/ cells and tested them again approximately 3 months later (67 days after ablation). At that time, PB had 82.4% ± 1.4% 4+ cells, not significantly different from earlier determinations. These experiments suggested that no significant populations of unablated 4f/f existed before our ablation, presumably because they were out-competed earlier by normal cells. Technical issues of nonablation (related to poly(I)-poly(C)) use) were excluded by successful ablation in other concurrent control animals. To solidify contributions of 4+/+ competitor cells, secondary transplantations were carried out at the time of euthanization. secondary recipients (n = 5) analyzed 4 months later showed 86.9 % ± 0.9% 4+ cells in the BM, 78.5% ± 1.8% in the spleen, and 80.7% ± 3.8% in PB. Repopulation by 4/ bone marrow or peripheral blood cells To test whether 4/ bone marrow cells can establish long-term reconstitution in the absence of competitor cells, mice received transplants of either 4/ or of a similar number of 4+/+ bone marrow cells per recipient (Table 1). Engraftment was assessed at 2, 10, 16, and 56 weeks later. To overcome homing problems and to test the role of the spleen in 4/ short-term hematopoiesis, we also used splenectomized recipients (1 and 5 x 106 4/ cells per recipient). The data were compared to 1 x 106 4+/+ concurrent control transplantations. As seen in Tables 1 and 2, short-term engraftment by 4/ cells at 2 weeks was less than 6% and less than 20% of that seen in recipients of control BM splenectomized or not, respectively. Thus, engraftment in the absence of the spleen, where many 4/ cells home and subsequently differentiate, is more severely impaired. Of interest is the peripheral blood leukocytosis in 4/ recipients (Tables 1, 2), which does not reflect higher engraftment but only the increased release of 4/ cells in circulation. View this table: [in this window] [in a new window]   Table 1.. Transplantation of BM 4/ cells to lethally irradiated 4+/+ nonsplenectomized recipients   View this table: [in this window] [in a new window]   Table 2.. Transplantation of BM 4/ cells to lethally irradiated 4+/+ splenectomized recipients   In the long-term donor reconstitution experiments, by 10 to 16 weeks bone marrow cellularity and progenitor content recovered to levels seen in concurrent controls (Table 1). This recovery had the hallmarks of 4/ hematopoiesis before transplantation: increase in progenitors in circulation and in spleen. However, detailed evaluation of 3 mice at 10 weeks showed that kit+/4+ were 46.4% in one of them, and in their spleens all 3 animals showed a significant increase in total 4+ cells (24.5%, 11.2%, and 9.6%) and among kit+ cells (31.3%, 37.6%, and 55.7%). Observations in the cohort of splenectomized recipients given the same number of 4/ cells and evaluated at 10 weeks after transplantation also showed an increase in 4+ cells (7 recipients, 16.3% ± 3.5%) and no increase in circulating progenitors compared to nonsplenectomized recipients (118 ± 36.9/mL vs 564 ± 46.4, Table 1). In contrast to data at 10 to 16 weeks (Table 1), hematopoiesis in the 3 mice surviving longer (56 weeks) was not maintained at levels seen at 10 to 16 weeks and was not increased further (ie, by BM-cell and progenitor numbers) with age, as seen in mice that did not receive transplants.15 (Total progenitor [CFU-C] content in BM, spleen, and PB was 815 789 ± 54 705 in 50-week-old 4+/+ animals [n = 4] and 1 363 601 ± 144 885 in 4/ animals who did not receive transplants [n = 4] of the same age. The 3 survivors had 195 487 ± 79 380 total CFU-Cs.) Unfortunately, at this time, controls that received transplants were not available for meaningful comparisons. To assess the self-renewal capacity of 4/ HSCs, we carried out secondary transplantation experiments, both at 10 weeks (ie, at the recovery phase) and at 56 weeks after transplantation. As indicated in the previous paragraph, one of the 3 mice evaluated at 10 weeks had approximately 50% 4+ cells in circulation, whereas the other 2 had mostly 4/ cells in circulation (Table 3, Figure 5). The latter 2 mice served as donors for five secondary recipients (1 x 106/recipient), and the one pseudo-chimeric mouse served as a donor for another 5 recipients. 4+/+ cells from control primary recipients also were transplanted in five secondary hosts. All recipients were evaluated about 4 months later. The data are shown in Table 3. At that time, a further increase in 4+ hematopoiesis and inferior BM CFU-C content in the recipients of 4/ cells are seen, compared to concurrent BM controls. The data collectively suggest that 4/ donor cells from primary recipients with phenotypically restored hematopoiesis have an inferior subsequent reconstitution capacity, as indicated by emergence of surviving host hematopoietic cells and by comparing them with concurrent controls. At 56 weeks the 3 surviving mice served as donors for ten secondary recipients (Figure 5). Evaluation of 9 of these recipients 26 weeks later showed that 5 had 33.6% ± 10.5% 4+ cells in BM, and the other 4 had 6.8% ± 1.9% 4+ cells. Each of the latter 4 mice with mostly 4/ cells served as donors for five tertiary recipients (20 total, Figure 5A). Twenty-five weeks later, there was only a 30% survival in this group (Figure 5B). The 6 surviving mice had 66.2% ± 14.1% 4+ cells in the blood. This represented host hematopoiesis (absence of neo in 4 mice [nos. 1-4, Figure 5C], indicating WT cells), whereas presence of neo in the other 2 (nos. 5-6, Figure 5C) reflected the recipient genotype (4f/f) and not unablated 4f/f donor cells. Taken together, our data indicate failing HSC self-renewal activity in the absence of 4 integrin. View this table: [in this window] [in a new window]   Table 3.. Secondary transplants (tested 4 months later) from primary recipients of 4/ BM cells (at 10 weeks after primary transplantation)   In addition to BM, PB cells from 4/ mice were transplanted into lethally irradiated recipients to test for LTRCs in the circulation of these mice. For this purpose, blood from 4+/+ or 4/ mice was given to 10 irradiated recipients from each group (0.3 mL/recipient). By day 30 all control recipients died; however, 3 of the recipients of 4/ survived beyond 90 days (Figure 6). These recipients were killed 6 months later for evaluation and showed greatly diminished hematopoiesis (ie, decreased CFU-C content in BM, no increase of progenitors in circulation or in spleen; data not shown) similar to features seen long term from BM 4/ cells. These results unequivocally suggest that the number of long-term repopulating cells is increased in circulation of 4/ animals compared to control animals, since despite their putative homing defect, they can establish long-term hematopoiesis in primary recipients.    Discussion Top Abstract Introduction Materials and methods Results Discussion References   Our previous studies in mice with 4 integrin gene ablation during adult life have shown no quantitative abnormalities in hematopoiesis during homeostasis for more than a year.15 However, there were aberrations in biodistribution of hematopoietic progenitors, which remained elevated in peripheral blood and were sequestered in the spleen. Furthermore, 4/ BM cells had a partially impaired homing when given to irradiated recipients, confirming many prior studies using antifunctional 4 antibodies.18 In addition, there was a significant deficiency in short-term engraftment during the first few weeks after transplantation, suggesting impairment in expansion of hematopoiesis under stress, in agreement with independent studies of post-5FU recovery15 and engraftment studies with anti-4-treated cells delivered intrafemorally.19 As the role of 4 integrins in the establishment and maintenance of long-term hematopoiesis was not clear from prior data, the present studies were undertaken. View larger version (40K): [in this window] [in a new window]   Figure 5.. Transplantation of 4- (/) BM or 4- (/) PB cells in lethally irradiated recipients. (A) Transplantation outcomes of mice given 4/ BM donor cells killed at 10 or 56 weeks after transplantation (Table 1). Secondary transplantations (Table 3) were done at 10 or 56 weeks after primary transplantation and were tested 16 and 26 weeks after secondary transplantation (white mice, < 10% 4+ cells; gray mice, > 10% 4+ cells; black mice, dead). Note the tendency in all secondary transplant recipients for reconstitution by surviving 4+ cells from hosts (gray mice). In tertiary transplants, there was only 30% survival (B), and the 6 surviving mice (no. 1-6 in Figure 5C) were reconstituted mostly (mice no. 1-4) by host cells that were neo-negative (= WT). Two mice (no. 5, 6) showing neo+ in panel C were not reconstituted by unablated 4f/f cells, but by host cells, since these recipients were f/f/cre(-)/neo+ mice.   View larger version (15K): [in this window] [in a new window]   Figure 6.. Survival of mice that received transplants of 0.3 mL PB from 4+/+ or 4-/- (/) mice. All recipients of 4+/+ blood died early, whereas one third of 4-/- PB recipients survived long term.   4-deficient donor cells display a competitive disadvantage in repopulation experiments All experiments in which 4/ cells were transplanted together with 4+/+ cells (at a ratio of 1:1 in 2 experiments and 3:1 test:competitor cells in one experiment) showed an early advantage of 4+/+ competitor cells over 4/ test cells. Such an early competitive advantage likely relies, at least in part, on the better homing of 4+/+ short-term repopulating cells (CFU-Cs) compared to 4/ ones, as documented in earlier studies.15 In addition to the inferior homing, impairment in the early expansion of 4/ stem and progenitor cells after homing, also documented previously and further supported by post-5FU recovery data, could certainly compound the homing deficit. However, if the competitive disadvantage is consequent only to the impaired homing and early expansion of 4/ donor cells, one would expect a stable contribution to hematopoiesis thereafter, as occurs with normally homing Rac2/CFU-C cells20 or stem-cell leukemia (SCL) conditionally ablated cells,21 or even a progressive contribution by 4/ LTRCs, if these did not have any homing/retention defect. Instead, what was seen was a progressive contribution by 4+/+ competitor cells, as indicated by peripheral-blood evaluations comparing early and late times after transplantation (Figures 2, 3) and by differences between bone marrow and peripheral blood at the time of killing. Such a trend seen in all 3 experiments is compatible neither with a normal HSC homing nor with functionally normal HSC behavior, at least in the presence of normal competitor cells. An analogous late decline, albeit of a smaller magnitude, was seen in chimera created with DBA/2 and C57BL/6 mice and was attributed to genetic differences in stem-cell-renewal activity between the 2 murine strains.22 Furthermore, the competitive repopulation outcome is reminiscent of data with chimeric mice (from 4-/- ES cells), in which there was no contribution to hematopoiesis by 4-/- cells beyond the first month of postnatal life.13,14 These chimeric data and our data with adult cells emphasize, therefore, the competitive disadvantage of 4-deficient HSCs in the presence of normal competitors. A similar competitive disadvantage for adult HSCs was seen in chimeras with Tie2-/-23 and Sca-1-/-24 cells, whereas SCL/ adult cells were competent in contrast to fetal ones,25 emphasizing regulatory differences between fetal and adult stem cells. It is important to point out that in the case of 4- or Tie2-deficient cells, there were no documented adverse effects on proliferation or differentiation of isolated hematopoietic cells that could complicate evaluation of engraftment rates seen in other competitive repopulations (ie, of PU.1-26 or Rac-deficient cells27). It was of interest that on rare occasions in the competitive repopulating experiments (Figure 3C), there was a long-term contribution by 4/ donor cells. Such a result raised the question of whether indeed donor 4/ can establish and support on their own long-term hematopoiesis in irradiated hosts. This issue was further explored by transplantations of bone marrow or peripheral blood from 4/ mice. Donor 4