B cell positive selection

Abstract: Rac1 and Rac2 GTPases transduce signals from multiple receptors leading to cell migration, adhesion, proliferation, and survival. In the absence of Rac1 and Rac2, B cell development is arrested at an IgD− transitional B cell stage that we term transitional type 0 (T0). We show that T0 cells cannot enter the white pulp of the spleen until they mature into the T1 and T2 stages, and that this entry into the white pulp requires integrin and chemokine receptor signaling and is required for cell survival. In the absence of Rac1 and Rac2, transitional B cells are unable to migrate in response to chemokines and cannot enter the splenic white pulp. We propose that loss of Rac1 and Rac2 causes arrest at the T0 stage at least in part because transitional B cells need to migrate into the white pulp to receive survival signals. Finally, we show that in the absence of Syk, a kinase that transduces B cell antigen receptor signals required for positive selection, development is arrested at the same T0 stage, with transitional B cells excluded from the white pulp. Thus, these studies identify a novel developmental checkpoint that coincides with B cell positive selection.

Abstract: It is well established that autoreactive B cells undergo negative selection. This stands in paradox with the high frequency of so-called natural autoreactive B cells producing low affinity polyreactive autoantibodies with recurrent specificities, suggesting that these B cells are selected on the basis of their autoreactivity. We previously described two transgenic mouse lines (with and without IgD) producing a human natural autoantibody (nAAb) that binds ssDNA and human Fcγ. In the absence of human IgG, nAAb-transgenic B cells develop normally. By crossing these mice with animals expressing knockin chimeric IgG with the human Fcγ, we now show that the constitutive expression of chimeric IgG promotes the increase of nAAb-expressing B cells. This positive selection is critically dependent on the presence of IgD, occurs in the spleen, and concerns all mature B cell subsets, with a relative preferential enrichment of marginal zone B cells. These data support the view that soluble self-Ags can result in positive clonal selection.

Abstract: Maturation of B cells within germinal centers (GCs) generates diversified B cell pools and high-affinity B cell antigen receptors (BCRs) for pathogen clearance. Increased receptor affinity is achieved by iterative cycles of T cell–dependent, affinity-based B cell positive selection and clonal expansion by mechanisms hitherto incompletely understood. Here we found that, as part of a physiologic program, GC B cells repressed expression of decay-accelerating factor (DAF/CD55) and other complement C3 convertase regulators via BCL6, but increased the expression of C5b-9 inhibitor CD59. These changes permitted C3 cleavage on GC B cell surfaces without the formation of membrane attack complex and activated C3a- and C5a-receptor signals required for positive selection. Genetic disruption of this pathway in antigen-activated B cells by conditional transgenic DAF overexpression or deletion of C3a and C5a receptors limited the activation of mechanistic target of rapamycin (mTOR) in response to BCR–CD40 signaling, causing premature GC collapse and impaired affinity maturation. These results reveal that coordinated shifts in complement regulation within the GC provide crucial signals underlying GC B cell positive selection. Heeger and colleagues report that activated B cells dynamically regulate the expression of complement regulatory proteins via the transcription factor BCL6. C3 convertase activity and C3aR1–C5aR1 signaling were both necessary for optimal B cell activation and germinal center formation.

Abstract: Memory B cells (MBCs) are key for protection from reinfection. However, it is mechanistically unclear how germinal center (GC) B cells differentiate into MBCs. MYC is transiently induced in cells fated for GC expansion and plasma cell (PC) formation, so-called positively selected GC B cells. We found that these cells coexpressed MYC and MIZ1 (MYC-interacting zinc-finger protein 1 [ZBTB17]). MYC and MIZ1 are transcriptional activators; however, they form a transcriptional repressor complex that represses MIZ1 target genes. Mice lacking MYC-MIZ1 complexes displayed impaired cell cycle entry of positively selected GC B cells and reduced GC B cell expansion and PC formation. Notably, absence of MYC-MIZ1 complexes in positively selected GC B cells led to a gene expression profile alike that of MBCs and increased MBC differentiation. Thus, at the GC positive selection stage, MYC-MIZ1 complexes are required for effective GC expansion and PC formation and to restrict MBC differentiation. We propose that MYC and MIZ1 form a module that regulates GC B cell fate.