Blood IMMUNOBIOLOGY

Cell-cell cooperation at the T helper cell/mast cell immunological synapse

Thu, 03 Dec 2009 09:02:29 PST

It has been suggested that mast cells might serve, under certain circumstances, as antigen-presenting cells (APCs) for T cells. However, whether cognate interactions between mast cells and class II–restricted CD4⁺ T cells actually occur is still an open question. We addressed this question by using peritoneal cell–derived mast cells (PCMCs) and freshly isolated peritoneal mast cells as APC models. Our results show that in vitro treatment of PCMCs with interferon- and interleukin-4 induced surface expression of mature major histocompatibility complex class II molecules and CD86. When interferon-/interleukin-4–primed PCMCs were used as APCs for CD4⁺ T cells, they induced activation of effector T cells but not of their naive counterparts as evidenced by CD69 up-regulation, proliferation, and cytokine production. Confocal laser scanning microscopy showed that CD4⁺ T cells formed immunological synapses and polarized their secretory machinery toward both antigen-loaded PCMCs and freshly isolated peritoneal mast cells. Finally, on cognate interaction with CD4⁺ T cells, mast cells lowered their threshold of activation via FcRI. Our results show that mast cells can establish cognate interactions with class II–restricted helper T cells, implying that they can actually serve as resident APCs in inflamed tissues.

B-cell follicle development remodels the conduit system and allows soluble antigen delivery to follicular dendritic cells

Bajenoff, M., Germain, R. N. — Thu, 03 Dec 2009 09:02:29 PST

Afferent lymph is transported throughout lymph nodes (LNs) by the conduit system. Whereas this conduit network is dense in the T-cell zone, it is sparse in B-cell follicles. In this study, we show that this differential organization emerges during lymph node development. Neonatal LNs lack B follicles, but have a developed T-cell zone and a dense conduit network. As new T and B cells enter the developing LN, the conduit network density is maintained in the T, but not the B zone, leading to a profound remodeling of the follicular network that nevertheless maintains its connectivity. In adults, the residual follicular conduits transport soluble antigen to deep regions, where follicular dendritic cells are abundant and appear to replace the fibroblastic reticular cells that enwrap conduits in the T zone. This strategic location correlates with the capacity of the follicular dendritic cells to capture antigen even in the absence of antigen-specific antibodies. Together, these results describe how the stromal organization of the T and B regions of LNs diverges during development, giving rise to distinct antigen transport and delivery modes in the 2 compartments.

Appearance of peripheral blood plasma cells and memory B cells in a primary and secondary immune response in humans

Thu, 03 Dec 2009 09:02:29 PST

In humans, the kinetics of the appearance of memory B cells and plasma cells during primary immunization are not well defined. In this study, we assessed the primary B-cell response of rabies-antigen naive volunteers during a 3-dose course of rabies vaccine compared with the B-cell response to a booster dose of rabies vaccine given to previously immunized volunteers. After a single dose of vaccine, in the naive group plasma and memory B cells appeared later (peak at day 10) than in the primed group (peak at day 7) and were at lower frequency. The most rapid responses (day 4) were detected after a third immunization in the naive group. This is the first study to document the detailed kinetics of the plasma cell and memory B-cell responses to immunization in adult humans and to demonstrate differences in the responses that relate to the preexisting immune status of the persons.

ATG-induced expression of FOXP3 in human CD4+ T cells in vitro is associated with T-cell activation and not the induction of FOXP3+ T regulatory cells

Broady, R., Yu, J., Levings, M. K. — Thu, 03 Dec 2009 09:02:29 PST

Several recent reports have suggested that in vitro exposure of CD4⁺ T cells to rabbit antithymocyte globulin (rATG), which is commonly used to prevent and treat graft-versus-host disease and allograft rejection, is an effective method to induce CD4⁺CD25⁺FOXP3⁺ T regulatory cells (Tregs). We and others, however, have shown that FOXP3 is also expressed in activated T cells. We therefore investigated whether the induction of FOXP3 expression by rATG resulted in a stable population of suppressive Tregs. We found that exposure of peripheral blood mononuclear cells (PBMCs) or conventional T cells to rATG resulted in induction of transient rather than stable expression of CD25 and FOXP3. Furthermore, rATG-treated T effector cells acquired neither an immunosuppressive profile of cytokine production nor suppressive capacity, even at the time of maximal FOXP3 expression. These findings indicate that the notion that rATG can be used to induce Tregs in vitro for cellular therapy in vivo should be re-evaluated.

Functionally distinct subsets of human NK cells and monocyte/DC-like cells identified by coexpression of CD56, CD7, and CD4

Thu, 26 Nov 2009 09:02:00 PST

The lack of natural killer (NK) cell–specific markers, as well as the overlap among several common surface antigens and functional properties, has obscured the delineation between NK cells and dendritic cells. Here, novel subsets of peripheral blood CD3/14/19^neg NK cells and monocyte/dendritic cell (DC)–like cells were identified on the basis of CD7 and CD4 expression. Coexpression of CD7 and CD56 differentiates NK cells from CD56⁺ monocyte/DC-like cells, which lack CD7. In contrast to CD7⁺CD56⁺ NK cells, CD7^negCD56⁺ cells lack expression of NK cell–associated markers, but share commonalities in their expression of various monocyte/DC-associated markers. Using CD7, we observed approximately 60% of CD4⁺CD56⁺ cells were CD7^neg cells, indicating the actual frequency of activated CD4⁺ NK cells is much lower in the blood than previously recognized. Functionally, only CD7⁺ NK cells secrete gamma interferon (IFN) and degranulate after interleukin-12 (IL-12) plus IL-18 or K562 target cell stimulation. Furthermore, using CD7 to separate CD56⁺ NK cells and CD56⁺ myeloid cells, we demonstrate that unlike resting CD7⁺CD56⁺ NK cells, the CD7^negCD56⁺ myeloid cells stimulate a potent allogeneic response. Our data indicate that CD7 and CD56 coexpression discriminates NK cells from CD7^negCD56⁺ monocyte/DC-like cells, thereby improving our ability to study the intricacies of NK-cell subset phenotypes and functions in vivo.

Alternatively activated macrophages engage in homotypic and heterotypic interactions through IL-4 and polyamine-induced E-cadherin/catenin complexes

Thu, 19 Nov 2009 09:02:19 PST

Alternatively activated macrophages (AAMs), triggered by interleukin-4 (IL-4) and IL-13, play a modulating role during Th2 cytokine-driven pathologies, but their molecular armament remains poorly characterized. Here, we established E-cadherin (Cdh1) as a selective marker for IL-4/IL-13–exposed mouse and human macrophages, which is STAT6-dependently induced during polarized Th2 responses associated with Taenia crassiceps helminth infections or allergic airway inflammation. The IL-4–dependent, arginase-1/ornithine decarboxylase–mediated production of polyamines is important for maximal Cdh1 induction, unveiling a novel mechanism for IL-4–dependent gene transcription. At the macrophage surface, E-cadherin forms a functional complex with the catenins that accumulates at sites of cell contact. Macrophage-specific deletion of the Cdh1 gene illustrates the implication of E-cadherin in IL-4–driven macrophage fusion and heterotypic interactions with CD103⁺ and KLRG1⁺ T cells. This study identifies the E-cadherin/catenin complex as a discriminative, partly polyamine-regulated feature of IL-4/IL-13–exposed alternatively activated macrophages that contributes to homotypic and heterotypic cellular interactions.

Homeostasis of dendritic cells in lymphoid organs is controlled by regulation of their precursors via a feedback loop

Thu, 12 Nov 2009 09:38:05 PST

Dendritic cells (DCs) are key coordinators of the immune response, governing the choice between tolerance and immunity. Despite their importance, the mechanisms controlling the size of the DC compartment are largely unknown. Using a mouse model allowing continuous DC depletion, we show that maintenance of DC numbers in spleen is an active process mediated by Flt3-L–dependent regulation of precursor differentiation into DCs, rather than by changes in proliferation of the differentiated DCs. In particular, the frequency and differentiation potential of intrasplenic DC precursors increased in response to reduced DC numbers. Levels of Flt3-L, a cytokine required for DC differentiation, increased in the blood after DC depletion and returned to normal levels once the DC compartment filled up again. Our data suggest a feedback regulation of DC homeostasis whereby reduction of the DC pool size promotes differentiation of their precursors, via increased Flt3-L availability. This mechanism is different to those known for other immune cell types, such as the B- and T-cell compartments, whereby lymphopenia induces proliferation of already differentiated lymphocytes.

CD56+ human blood dendritic cells effectively promote TH1-type {gamma}{delta} T-cell responses

Gruenbacher, G., Gander, H., Rahm, A., Nussbaumer, W., Romani, N., Thurnher, M. — Thu, 12 Nov 2009 09:38:05 PST

CD56⁺ human dendritic cells (DCs) have recently been shown to differentiate from monocytes in response to GM-CSF and type 1 interferon in vitro. We show here that CD56⁺ cells freshly isolated from human peripheral blood contain a substantial subset of CD14⁺CD86⁺HLA-DR⁺ cells, which have the appearance of intermediate-sized lymphocytes but spontaneously differentiate into enlarged DC-like cells with substantially increased HLA-DR and CD86 expression or into fully mature CD83⁺ DCs in response to appropriate cytokines. Stimulation of CD56⁺ cells containing both DCs and abundant T cells with zoledronate and interleukin-2 (IL-2) resulted in the rapid expansion of T cells as well as in IFN-, TNF-, and IL-1β but not in IL-4, IL-10, or IL-17 production. IFN-, TNF-, and IL-1β production were almost completely abolished by depleting CD14⁺ cells from the CD56⁺ subset before stimulation. Likewise, depletion of CD14⁺ cells dramatically impaired T-cell expansion. IFN- production could also be blocked by neutralizing the effects of endogenous IL-1β and TNF-. Conversely, addition of recombinant IL-1β, TNF-, or both further enhanced IFN- production and strongly up-regulated IL-6 production. Our data indicate that CD56⁺ DCs from human blood are capable of stimulating CD56⁺ T cells, which may be harnessed for immunotherapy.

The thymus-independent immunity conferred by a pneumococcal polysaccharide is mediated by long-lived plasma cells

Thu, 12 Nov 2009 09:38:05 PST

It was recently shown that bacterial thymus-independent (TI) antigens confer long-lasting immunity and generate memory B lymphocytes. However, reactivation of TI memory B cells is repressed in immunocompetent mice, thus raising the issue of the mechanism whereby TI vaccines confer immune protection. Here, we propose an explanation to this apparent paradox by showing that a Streptococcus pneumoniae capsular polysaccharide (PS) generates long-lived bone marrow (BM) plasma cells which frequency can be increased by CpG oligodeoxynucleotides (ODNs). The adjuvant effect of CpG ODNs on the PS3 Ab response is directly targeted to B cells and does not involve B-1a cells. We also demonstrated that BM plasma cells generated in response to the thymus-dependent (TD) form of the PS vaccine have a higher secretion capacity than those produced after immunization with the CpG-adjuvanted PS vaccine. Finally, we show that the PS-specific BM plasma cell compartment is sufficient to confer full protection of vaccinated mice against S pneumoniae infection. Altogether, our results show that TI antigens like their TD counterparts can generate both the lymphoid and the plasma cell component of B-cell memory. They also provide a framework for the improvement and widespread usage of TI vaccines.