CD4 and systemic lupus erythematosus: In the cGVHD lupus model, the key cellular mechanism that results in the loss of B-cell tolerance is the interaction of donor bm12 CD4+ T cells with MHC class II on host B6 B-cell surface, inducing high levels of circulating anti-nuclear antibodies, concomitantly with large frequencies of T follicular helper (Tfh) cells, germinal center (GC) B cells, and plasma cells (18, 19).