DLL1 and rheumatoid arthritis: Like what was described for T cells above, Dll1 provided by murine FRC promotes MZ B cell survival and plasma cell differentiation (Figure 4E).291 MZ B cells express both classical (MHC) and non-classical (CD1 family) antigen presentation machinery.291,397–399 MZ-like B cells expressing CD1 family transcripts are found in RA synovial tissue.400 Thus, FRC- or FDC-like fibroblasts in the RA synovium may provide DLL1/Notch2 signals that promote MZ-like B cell survival and plasma cell differentiation in this inflamed tissue.