Antigen presentation is a function of all B cell subpopulations separated on the basis of size

Purified splenic B cells from nonimmune mice were separated by counterflow centrifugal elutriation into 6 subpopulations containing cells of discrete sizes ranging from 119 to 200 μm3. B cells of each subpopulation were competent to process and present a native globular protein antigen, cytochrome c, to a cytochrome c‐specific T cell hybrid. In all cases, the B cells' antigen‐presenting function was radiation sensitive and did not require T cells or T cell products, since B cells fixed with paraformaldehyde effectively presented a carboxyl‐terminal peptide fragment of cytochrome c containing the T cell determinant. Furthermore, the antigen‐presenting function of B cells of each subpopulation was augmented by treatment with submitogenic doses of the F(ab')2 fragment of rabbit anti‐mouse Ig antibodies, in that 10‐30‐fold fewer B cells were required and higher maximal T cell responses were achieved, indicating that B cells of all sizes are capable of being regulated in their antigen presentation function through their surface Ig. In addition, B cells of each subpopulation responded to soluble factors present in the supernatants of activated T cells as evidenced by an increase in volume and by the uptake of [3H]thymidine. These results indicate that B cells, regardless of size, are able to participate in at least two essential phases of T cell‐dependent antibody responses, initiating the interaction by processing and presenting antigen to helper T cells and responding to soluble helper factors secreted by activated T cells.