The development and fate of follicular helper T cells defined by an IL-21 reporter mouse.

Germinal centers require CD4⁺ follicular helper T cells (TFH cells), whose hallmark is expression of the transcriptional repressor Bcl-6, the chemokine receptor CXCR5 and interleukin 21 (IL-21). To track the development and fate of TFH cells, we generated an IL-21 reporter mouse by introducing sequence encoding green fluorescent protein (GFP) into the Il21 locus; these mice had expression of IL-21­GFP in CD4⁺CXCR5⁺PD-1⁺ TFH cells. IL-21­GFP⁺ TFH cells were multifunctional helper cells that coexpressed several cytokines, including interferon-g (IFN-g), IL-2 and IL-4. TFH cells proliferated and gave rise to transferrable memory cells with plasticity, which differentiated after recall into conventional effector helper T cells and TFH cells. Thus, we demonstrated that TFH cells were not terminally differentiated but instead retained the flexibility to be recruited into other helper T cell subsets and nonlymphoid tissues.

Engagement of CD83 on B cells modulates B cell function in vivo.

The transmembrane glycoprotein CD83 is an important regulator of both thymic T cell maturation and peripheral T cell response. Recent studies suggested that CD83 is also involved in the regulation of B cell maturation, activation, and homeostasis. In this study, we show that in vivo overexpression of CD83 dose dependently interfered with the Ig response to thymus-dependent and thymus-independent model Ag immunization. CD83 deficiency, in contrast, which was restricted to B cells in mixed bone marrow chimeras, led to unchanged or even slightly increased Ig responses. Strikingly, the engagement of CD83 that is naturally up-regulated on wild-type B cells by injection of anti-CD83 mAb in vivo induced a 100-fold increase in the IgG1 response to immunization. Kinetic analysis revealed that CD83 had to be engaged simultaneously or shortly after the B cell activation through injection of Ag, to modulate the IgG1 secretion. Furthermore, using mixed bone marrow chimeras in which either selectively the B cells or the dendritic cells were CD83 deficient, we demonstrate that anti-CD83 mAb mediated its biologic effect by engaging CD83 on B cells and not on CD11c(+) dendritic cells. Taken together, we provide strong evidence that CD83 transduces regulatory signals into the very B cell on which it is expressed.

CD83 regulates splenic B cell maturation and peripheral B cell homeostasis.

The central function of murine CD83 that is expressed on thymic epithelial cells is to induce the progression of double-positive thymocytes to single CD4-positive T cells. Several lines of evidence suggest an additional role for CD83 in the regulation of peripheral T and B cell responses. Here we show that CD83 is expressed by immature B cells and regulates their further maturation and survival in the periphery. Employing mixed bone marrow chimeras, we compare wild-type, CD83 over-expressing and CD83-deficient B cells within the same host. CD83 over-expression on the immature B cells themselves led to an accumulation of transitional B cells and a reciprocally reduced maturation of follicular B cells that was strictly correlated to the intensity of CD83 over-expression. The absence of CD83 on B cells resulted in a decreased maturation of marginal zone B cells and conferred a mild selection advantage for B cell survival in the periphery. Consenting with these findings, the over-expression of CD83 specifically and dose dependently interfered with homeostasis of B cells while T cell survival was not affected by CD83 over-expression over a period of 30 weeks. Taken together, our data suggest that CD83 negatively regulates B cell maturation and survival.

CD83 on murine APC does not function as a costimulatory receptor for T cells.

The transmembrane glycoprotein CD83 is rapidly upregulated on murine and human DC upon maturation and therefore a costimulatory function for T cell activation has been suggested. Studies employing human APC indeed showed that CD83 expression was positively correlated to the stimulatory capacity of the APC. Murine APC that were CD83 deficient however, did not display a reduced capacity to activate T cells. To elucidate this contradiction, we thoroughly compared the stimulatory capacity of CD83-overexpressing and CD83-deficient APC. Here we show that CD83 expression levels on APC did not affect the capacity of the APC to activate CD8(+) T cells. CD83 expression levels did not significantly affect CD4(+) T cell activation in vivo, but a weak positive correlation of CD83 expression with CD4(+) T cell activation was observed in vitro under suboptimal stimulation conditions. As CD83 expression also positively correlated with MHC-II but not with MHC-I expression, this differential stimulation specifically of CD4(+) T cells could be explained by a higher density of MHC-II peptide complexes on the APC surface. Taken together, our results strongly suggest that CD83 does not deliver crucial costimulatory signals to murine T cells.

Mcl-1 is essential for germinal center formation and B cell memory.

Lymphocyte survival during immune responses is controlled by the relative expression of pro- and anti-apoptotic molecules, regulating the magnitude, quality, and duration of the response. We investigated the consequences of deleting genes encoding the anti-apoptotic molecules Mcl1 and Bcl2l1 (Bcl-x(L)) from B cells using an inducible system synchronized with expression of activation-induced cytidine deaminase (Aicda) after immunization. This revealed Mcl1 and not Bcl2l1 to be indispensable for the formation and persistence of germinal centers (GCs). Limiting Mcl1 expression reduced the magnitude of the GC response with an equivalent, but not greater, effect on memory B cell formation and no effect on persistence. Our results identify Mcl1 as the main anti-apoptotic regulator of activated B cell survival and suggest distinct mechanisms controlling survival of GC and memory B cells.

CD83 modulates B cell function in vitro: increased IL-10 and reduced Ig secretion by CD83Tg B cells.

The murine transmembrane glycoprotein CD83 is an important regulator for both thymic T cell maturation and peripheral T cell responses. Recently, we reported that CD83 also has a function on B cells: Ubiquitous transgenic (Tg) expression of CD83 interfered with the immunoglobulin (Ig) response to infectious agents and to T cell dependent as well as T cell independent model antigen immunization. Here we compare the function of CD83Tg B cells that overexpress CD83 and CD83 mutant (CD83mu) B cells that display a drastically reduced CD83 expression. Correlating with CD83 expression, the basic as well as the lipopolysaccharide (LPS) induced expression of the activation markers CD86 and MHC-II are significantly increased in CD83Tg B cells and reciprocally decreased in CD83mu B cells. Wild-type B cells rapidly upregulate CD83 within three hours post BCR or TLR engagement by de novo protein synthesis. The forced premature overexpression of CD83 on the CD83Tg B cells results in reduced calcium signaling, reduced Ig secretion and a reciprocally increased IL-10 production upon in vitro activation. This altered phenotype is mediated by CD83 expressed on the B cells themselves, since it is observed in the absence of accessory cells. In line with this finding, purified CD83mu B cells displayed a reduced IL-10 production and slightly increased Ig secretion upon LPS stimulation in vitro. Taken together, our data strongly suggest that CD83 is expressed by B cells upon activation and contributes to the regulation of B cell function.

CD83 is a regulator of murine B cell function in vivo.

The transmembrane glycoprotein CD83 has been described as a specific maturation marker for dendritic cells and several lines of evidence suggest that CD83 regulates thymic T cell maturation as well as peripheral T cell activation. Here we show for the first time that CD83 is involved also in the regulation of B cell function. CD83 is up-regulated on activated B cells in vivo, specifically in the draining lymph nodes of Leishmania major-infected mice. The ubiquitous transgenic (Tg) expression of CD83 interferes with Leishmania-specific T cell-dependent and with T cell-independent antibody production. This defect is restricted to the B cell population since the antigen-specific T cell response of CD83Tg mice to L. major infection is unchanged. The defective immunoglobulin (Ig) response is due to Tg expression of CD83 on the B cells because wild-type B cells display normal antigen-specific responses in CD83Tg hosts and CD83Tg B cells do not respond to immunization in a mixed wild-type/CD83Tg bone marrow chimera. Finally, the treatment of non-Tg C57BL/6 mice with anti-CD83 mAb induces a dramatic increase in the antigen-specific IgG response to immunization, thus demonstrating a regulatory role for naturally induced CD83 on wild-type B cells.

Transgenic expression of a CD83-immunoglobulin fusion protein impairs the development of immune-competent CD4-positive T cells.

The murine transmembrane glycoprotein CD83 is an important regulator for both thymic T cell maturation and peripheral T cell response. CD83 deficiency leads to a block in the thymic maturation of CD4-positive T cells, and interference with peripheral CD83/CD83 ligand interaction by addition of soluble CD83 suppresses immune responses in vivo and in vitro. Here we report the generation of a mouse transgenic for a fusion protein consisting of the extracellular domain of murine CD83 fused to the constant part of human IgG1 heavy chain. Thymic selection of CD4-positive T cells was unchanged in CD83Ig transgenic and in CD83Ig/OT-2 double-transgenic mice. However, thymic and peripheral CD4-positive T cells derived from CD83Ig/OT-2 transgenic mice displayed a reduced cytokine response to antigenic stimulation in vitro, whereas CD83Ig/OT-1-derived CD8-positive T cells showed normal cytokine secretion. The T cell defect was relevant in vivo, since a sub-lethal infection with Trypanosoma cruzi led to an increased parasitemia and reduced survival rate of CD83Ig transgenic mice compared to wild-type C57BL/6 mice. In contrast, in vivo application of recombinant CD83Ig did not result in an increase in parasitemia. Taken together our data suggest that thymic selection in the presence of CD83Ig leads to an intrinsic T cell defect of CD4-positive T cells resembling the phenotype described for CD4-positive T cells derived from CD83-deficient mouse strains.