B Cells from Mice Prematurely Expressing Human Complement Receptor Type 2 Are Unresponsive to T-Dependent Antigens

Abstract

Complement receptor type 2 (CR2/CD21), in association with CD19, plays an important role in enhancing mature B cell responses to opsonized Ags. We have shown that mice expressing a human CR2/CD21 (hCR2/CD21) transgene during the CD43+/CD25− late pro-B cell stage of B cell development demonstrate marked changes in subsequent B cell ontogeny. In the present study, we show that the humoral immune response to the T cell-dependent Ag, sheep RBC, is muted severely in a manner inversely proportional to B cell expression level of hCR2. Individual Ag-specific IgG isotypes vary in the degree to which they are affected but all are reduced while IgM titers are normal. A substantial reduction in germinal centers, both in size and frequency, in the spleens of immunized hCR2 transgenic mice demonstrates a failure to maintain germinal center reaction. However, both IgM expression levels and LPS-proliferative responses appear fully intact in B cells from hCR2-positive mice, suggesting that this alteration in B cell phenotype is different qualitatively from that of specific Ag-defined anergy models. These data suggest that the unresponsiveness to T-dependent Ags displayed by hCR2-positive B cells is linked to an increase in the level of stimulus required to propel the B cell into a fully activated state and thus a normal humoral immune response to Ags. We conclude that this phenotype and these mice may offer an additional means to dissect mechanisms underlying B cell tolerance and Ag responsiveness both in bone marrow and periphery.

Defective B cell ontogeny and immune response in human complement receptor 2 (CR2, CD21) transgenic mice is partially recovered in the absence of C3

“…We have previously shown that hCR2 expression levels drop significantly as the B cells mature in the bone marrow from B220 lo to B220 hi and are reduced further still as B cells migrate into the periphery (Birrell et al, 2005). However, on analysis of B cells isolated from C3 −/− hCR2 high mice, we found that this reduction in hCR2 expression levels was no longer fully apparent ( Figure 5A).…”

“…Clearly, lack of C3 does not completely reverse the phenotype or the reduction in B cell numbers noted in hCR2 high mice and thus, hCR2 binding to mouse C3 or its breakdown fragments cannot be the only factor in the observed defect. There is still evidence of a previously noted down turn in hCR2 expression levels as B cells mature into the peripheral tissue despite the absence of the C3 ( (Birrell et al, 2005); Figure 5A), suggesting the possibility that hCR2 interacts with other molecules as the B cell matures in the mouse. Evidence from analysis of myeloma cell adhesion to BM stroma cells has suggested that CR2 could interact with CD23 (Huang et al, 1995), and highlights one potential complement independent means of CR2 engagement.…”

“…The generation of mice expressing human CR2 under the control of a lambda light chain promoter-enhancer mini gene has resulted in mice that have an altered B cell ontogeny and a severely muted response to foreign or self antigen (Birrell et al, 2005;Marchbank et al, 2002). Through the reduction or removal of signaling capability of the CD19/CR2 complex by cross breeding the CD19 −/− mice with the hCR2 tg mice, we hoped to remove a negative signal we envisaged was being delivered as a consequence of hCR2 interacting with CD19, after its ligation on the B cell surface, during early B cell development in the hCR2 high mice.…”

“…In contrast to mice expressing hCR2 under its own promoter, expression of hCR2 was detected during the early stages of B cell development and resulted in a partial block in B cell development with a subsequent 40-60% reduction in mature B cell numbers. Furthermore, despite association with mCD19 and ability to flux Ca 2+ , mature B cells in the lambda hCR2 tg mice failed to respond adequately in vivo when challenged with TD and TI antigens (Birrell et al, 2005;Kulik et al, 2007;Marchbank et al, 2002). Our recent analysis suggests that this is very likely due to a result of intrinsic changes in signaling pathways downstream of BCR and CR2/CD19 crosslinking.…”

“…We have established that mice prematurely expressing human CR2 (hCR2) in the B cell lineage have a defective B cell ontogeny and immune response (Birrell et al, 2005;Marchbank et al, 2002). We have also shown that signaling through hCR2, and presumably mouse CD19, can occur in immature B cells from hCR2 high mice (Kulik et al, 2007).…”