No abstract

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. Here we investigate 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) following immunization. This revealed Mcl1 and not Bcl2l1 to be indispensable for formation and persistence of germinal centers (GC). 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 antiapoptotic regulator of activated B cell survival and suggest distinct mechanisms controlling survival of GC and memory B cells.Vertebrate immune responses are characterized by the clonal expansion of antigen-specific lymphocytes, by their differentiation into effector cells and by the production of small, persistent populations of memory cells. An added feature of B cell immunity is the increasing affinity of the antibody response with time, with B cells expressing high affinity antigen receptors (BCR) preferentially recruited into the effector and memory compartments (1). The processes underpinning changes in affinity of immunoglobulin receptors occur within germinal centers (GC), transient structures that arise after T cell dependent immunization (2-4). Thus the factors governing the survival of GC B cells will determine the qualitative and quantitative attributes of the effector cells, which in this case are plasma cells, and memory B cells. Survival of GC B cells is mediated by both the intrinsic and extrinsic apoptotic cell death pathways (5) with roles proposed for FASL/FAS (6,7), Bcl2l1 (Bclx L ) (8), Bcl2 (9,10), and Bim (11). These studies, however, do not identify the prosurvival molecules that are physiologically relevant within the GC To address the components of the intrinsic apoptotic pathway regulating GC B cell behaviour, we first measured the expression of relevant proteins, comparing GC and follicular B cells (Fig. 1). Besides Bcl2l1 and Bim expression being up-regulated and Bcl-2 down-regulated, as previously reported (6,12,13), Mcl1 protein was increased in GC B cells (Fig. 1 (Fig. 3C). The frequency of NP-specific IgG1 ASC in both spleen and bone marrow was severely diminished in homozygous Mcl1 deleted mice with a corresponding reduction in heterozygous Mcl1 deleted mice (Fig. 3D). Serum titres of anti-NP IgG1, but not IgM, were correspondingly and significantly reduced ( fig. S5) (Fig. 4A and fig. S11), suggesting that entry into and persistence within the GC-derived memory compartment is not influenced by amounts of Mcl1. Other routes of memory B cell formation may have distinct survival requirements (25).It was possible that GC B cells lacking either Bcl2l1 or Mcl1 had compensated for a survival disadvantage by acquiring higher ...

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 majorinfected 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.

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.

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 upregulated 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.