Figure 1.

Four different types of T cell–B cell interaction can occur in the germinal center: (A) following antigen recognition the specific B cells express Fas and in time became capable of presenting antigen to antigen-specific T cells. Strong affinity B cells maintain their antigen specificity during somatic mutation and as a consequence the BCR transduces signals even during competition for the limited available antigen. The BCR-mediated survival signals compete with Fas-induced cell death. (B) The naive (bystander) B cells do not interact with activated, FasL-expressing T cells in the absence of presentation of the appropriate antigen. In this situation their Fas expression is also very limited. As mentioned above, these cells are not targets of Fas-induced cell death. (C) The low affinity B cells, and B cells which lost antigen-specificity because of somatic mutation, are first targets of Fas-mediated killing. Maintaining antigen presenting capacity, but losing antigen triggering in the competition for the antigen, these cells contact the T cells, but do not access the survival signal. This explains the elevated antibody level in lpr mice where the antigen non-specific B cells can survive. (D) Anergic B cells are the main targets of Fas-mediated killing; the antigen receptor signaling of anergic B cells is desensitized due to their permanent activation. This group can explain the enhanced autoantibody production observed in lpr mice.