ANTIGEN-SPECIFIC B-CELLS ARE REQUIRED FOR THE SECONDARY RESPONSE OF T-CELLS BUT NOT FOR THEIR PRIMING

We studied the potential role of B cells in T cell responses using sev ere-combined immunodeficient (SCID) mice grafted with the thymus from fetal C.B-17 mice (TG mice). These mice developed both CD4(+) and CD8( +) T cells, but not B cells within 2 months after transplantation. TG mice showed normal delayed-type hypersensitivity responses against the immunizing antigen ovalbumin (OVA). Lymph node (LN) cells of TG mice proliferated well in response to concanavalin A (Con A). Further, Con A stimulation induced the production of interleukin (IL)-2, IL-6 and i nterferon (IFN)-gamma and the expression of IL-4 mRNA. Thus, TG mice w ere reconstituted without remarkable immunodeficiency. However, these T cells failed to proliferate to OVA stimulation. Response to OVA was also inhibited in SCID mice grafted with fetal C.B-17 liver cells when B cells were depleted in the proliferation assay. Unresponsiveness ag ainst immunizing antigen was restored by the addition of antigen-prime d B cells, but not by naive B cells, lipopolysaccharide-activated B ce lls or B cells primed with sheep red blood cells. Next, we examined wh ether antigen-primed B cells could induce T cell responses without pro fessional antigen-presenting cells (APC). T and B cells were purified from OVA-immunized mice by cell sorter. These T cells proliferated in response to OVA and produced IFN-gamma in the absence of non-B APC. Wh en anti-CD80 or anti-CD86 was added in the assay, proliferation and IF N-gamma production was inhibited. These results indicate that B cells activated specifically with antigen are required for the secondary res ponse of T cells, but not for their priming.