The control of systemic infection by encapsulated microorganisms requires T-independent type II (TI-2) Ab responses to bacterial polysaccharides. To understand how such responses evolve, we explored the function of transmembrane activator calcium modulator and cyclophilin ligand interactor (TACI), a member of the TNFR family, required for TI-2 Ab production. Quasimonoclonal (QM) mice produce robust TI-2 responses to 4-hydroxy-3-nitrophenylacetate (NP)-Ficoll, owing to the high precursor frequency of NP-specific B cells in the marginal zone of the spleen. QM mice that lack TACI produce decreased numbers of IgM (2-fold) and IgG (1.6-fold) NP-specific ASCs, compared with TACI-positive QM mice in response to immunization with NP-Ficoll. Our studies indicate that TACI acts at a remote time from activation because TACI is not necessary for activation and proliferation of B cells both in vitro and in vivo. Instead, TACI-deficient QM B cells remained in the cell cycle longer than TACI-proficient QM cells and had impaired plasma cell differentiation in response to NP-Ficoll. We conclude that TACI has dual B cell-autonomous functions, inhibiting prolonged B cell proliferation and stimulating plasma cell differentiation, thus resolving the longstanding paradox that TACI may have both B cell-inhibitory and -stimulatory functions. By promoting plasma cell differentiation earlier during clonal expansion, TACI may decrease the chances of autoantibody production by somatic hypermutation of Ig genes in response to T-independent Ags.

Fitness of cell-mediated immunity is thought to depend on TCR diversity; however, this concept has not been tested formally. We tested the concept using JH−/− mice that lack B cells and have TCR Vβ diversity <1% that of wild-type mice and quasimonoclonal (QM) mice with oligoclonal B cells and TCR Vβ diversity 7% that of wild-type mice. Despite having a TCR repertoire contracted >99% and defective lymphoid organogenesis, JH−/− mice rejected H-Y-incompatible skin grafts as rapidly as wild-type mice. JH−/− mice exhibited T cell priming by peptide and delayed-type hypersensitivity, although these responses were less than normal owing either to TCR repertoire contraction or defective lymphoid organogenesis. QM mice with TCR diversity contracted >90%, and normal lymphoid organs rejected H-Y incompatible skin grafts as rapidly as wild type mice and exhibited normal T cell priming and normal delayed-type hypersensitivity reactions. QM mice also resisted Pneumocystis murina like wild-type mice. Thus, cell-mediated immunity can function normally despite contractions of TCR diversity >90% and possibly >99%.

Survival of mature B cells is thought to depend on the BCR signaling (BCR) because ablation of either H chain (HC) expression or BCR signaling causes B cells to rapidly disappear. Whether a complete BCR is required for survival of mature B cells is not known. To address this question, we generated a mouse in which we can repress the expression of a transgenic Ig L chain (IgL) by doxycycline (IgL-repressible mouse). Repression of IgL abrogated expression. Surprisingly, however, IgL-negative B cells survived longer than 14 wk, expressed signal-competent HC on the cell’s surface, and active unfolded protein response factors. Like postgerminal center B cells, IgL-negative B cells were small lymphocytes, not dividing and expressed Bcl-6. Our results indicate that expression of unpaired HC, as it may occur as a consequence of Ag ligation, somatic hypermutation, or receptor editing, facilitates the survival of cells either by inducing receptor signaling or by inducing unfolded protein response and/or the expression of survival genes such as Bcl-6.