Effects of a human urinary mitogen on subpopulations of peripheral blood mononuclear leukocytes

We have earlier isolated, to apparent homogeneity, a 27-28 kD human basic protein (UM) from the urine of a patient with myelomonocytic leukaemia. UM is a mitogen for resting human peripheral blood mononuclear leukocytes (PBML). We have now further defined the effect of UM on human PBML and their subpopulations in 6-day cultures. Cell proliferation was measured by 3H-thymidine uptake and Ig production by the plaque forming cell (PFC) response. Whole PBML responded to UM with proliferation and an increase in PFC. The PFC response was at best equal to and frequently synergistic with that produced by pokeweed mitogen and occurred in the three major Ig classes. To test the effect of UM on subpopulations of PBML, adherent cells (AC) were isolated by plastic adherence and T and B enriched populations by rosetting with sheep red blood cells. The proliferative response of T cells needed the presence of AC whilst the effect on Ig production by B cells required both T cell help and the presence of AC. Human thymocytes also responded to UM by proliferation. The results show that, in addition to being a T cell mitogen, UM is also a T cell dependent polyclonal B cell activator.