Small and large B cells respond differently to T cell-derived B cell growth and differentiation factors.

A major problem in B cell biology is the determination of the roles played by helper T (TH) cells vs cytokines in the activation, replication, and differentiation of B lymphocytes. There is general agreement that activated B cells in cycle can replicate and terminally differentiate when provided with appropriate T cell-derived lymphokines. There is considerable controversy, however, as to whether cytokines can induce resting G0 B cells to secrete IgM. Some reports claim that TH cells are required before cytokines can act. In contrast, other reports claim that some T cell-derived supernatants (SN) have the capacity to activate resting B cells to become immunoglobulin-secreting cells. In the present study, we have examined one such SN (S26.5) as well as the EL-4 and PK 7.1 SN for their capacity to activate resting G0 B cells and a population of less dense B cells containing activated cells. These two populations, separated by Percoll density centrifugation, were characterized for size, stage in the cell cycle, and cell surface phenotype. It was shown that the most dense population contained predominantly G0 B cells, whereas the less dense population contained a subset of cells in cycle. Our studies show that neither T cell SN nor, indeed, any combination of cytokines and anti-immunoglobulins caused a major increase in the number of cells secreting IgM in the population enriched in G0 cells. In contrast, the T cell SN caused marked increases in the generation of IgM secreting cells in the population that contained a large proportion of activated cells. Limiting dilution analysis confirmed that the number of responding cells in the dense cell population was substantially lower than the number of responding cells in the less dense population. The small number of precursors in the dense B cell population may be attributed to contamination of that population with cells that have undergone activation steps in vivo. The present results, therefore, add further evidence to an existing large body of evidence that TH cells are essential for the terminal differentiation of G0 resting B cells in response to thymus-dependent antigens.