Interleukin 5 (IL-5) can act in G0/G1 to induce S phase entry in B lymphocytes from normal and autoimmune strain mice and in transformed leukemic B cells

In addition to its ability to enhance antibody secretion, Interleukin 5 (IL-5) enhances murine B lymphocyte proliferation. This so-called growth factor activity has been amply demonstrated by many laboratories assessing thymidine incorporation or cell recovery. Attempts to actually quantitate the fraction of fresh splenic B cells responding to IL-5, by limiting dilution analysis or other means, with few exceptions have yielded disappointingly small numbers--generally between 1 and 5%, or perhaps less. We have recently identified the peritoneal cavity as a reservoir rich in IL-5-responsive B cells. In this report, we provide independent corroboration of this high IL-5 reactivity by means of cell cycle analysis. Low-density peritoneal B cells, more than 90% of which are in G0 and G1 phases, were stimulated with polyclonal activators in the presence of mitotic inhibitors. Frequencies of IL-5-responsive B cells were measured by observing the differences in the proportions of cultured cells entering S and later phases in the presence, compared to the absence, of IL-5. Some 10 to 20% more of the low-density peritoneal B cells from normal mice entered S phase when IL-5 was present with LPS + DXS. A similar IL-5-mediated elevation in the frequency of S phase entry was seen with peritoneal B cells from the autoimmune mouse strain NZB. Furthermore, a measurable fraction of peritoneal B cells from these mice were even capable of responding to IL-5 alone. These IL-5-induced increases could be blocked by anti-IL-5 mAb. About 30% of the BCL1 leukemic B cell line initiated DNA replication when stimulated with IL-5 alone. Hence, IL-5-responsive B cell fractions have been measured for some normal, autoimmune strain and transformed leukemic B cell phenotypes. In addition to quantitating the proportion of IL-5-responsive B cells, these experiments formally demonstrate that IL-5 can act in the G1 phase to increase S phase entry.