Regulation of germinal centers and follicular helper t cells by mhcii+ antigen presenting cells

Follicular helper T cells (TFH) are a CD4 helper T cell subset that provide help to B cells in the germinal center (GC), a specialized environment in which activated B cells undergo class switching and somatic hypermutation to generate high affinity antibody. GC initiation relies on both B cell activation by antigen, as well as CD4 T cell help including that provided by TFH . While much is known about the events that occur in the GC, we wished to further examine the cellular interactions that precede the GC, in order to better understand effective GC and TFH formation. In order to investigate the role(s) for DC and B cell MHC Class II (MHCII) antigen presentation in TFH and GC B cell differentiation, we utilized several mouse models in which MHCII expression is restricted to DCs and/or B cells and in addition, B cell MHCII expression can be temporally controlled. It has been suggested that TFH differentiation initiates early after T cell activation and is independent of cognate interactions with B cells. Consistent with this, MHCII expression restricted to CD11chi dendritic cells (DCs) drives differentiation of a TFH intermediate (termed pre-TFH). Pre-TFH express CXCR5, ICOS and the critical transcription factor Bcl-6; however, they do not secrete IL-21 and IL-4. We found that B cell restricted antigen presentation alone was insufficient for CD4+ T cell priming and TFH formation. However, the combination of DC and B cell restricted antigen presentation was sufficient for complete TFH differentiation. This suggests that DCs must initiate T FHdifferentiation, which is then completed by B cell antigen presentation. Induction of B cell MHCII expression after initial T cell priming revealed that complete TFH differentiation remained intact, indicating that cognate pre-TFH-B cell interactions can occur after the initiation of the immune response for complete TFH differentiation. Strikingly, antigen specific B cells failed to differentiate into GC B cells when cognate T-B interactions occur after the initiation of the immune response. Consequently we observed a decrease in affinity maturation and class switching. These data indicate that activated B cells have a narrow window after activation via the B cell receptor in which to receive cognate T cell help and adopt a GC fate. As a whole, the data presented here demonstrate the cooperative and reciprocal nature of T and B cell activation and subsequent differentiation into TFH and GCs and demonstrate that the tightly controlled nature of these processes