Stimulation of lymphoid cell proliferation by Mycoplasma orale, a common cell culture contaminant

Mycoplasma orale, maintained as a contaminant of a mouse hybrid cell line, induces an intense proliferation in short-term culture of lymphoid cells of inbred mice. Cell division induced by the contaminated cell culture fluid reaches a maximum on day four and declines rapidly thereafter. Culture fluids from hybrid cells freed of contamination do not cause proliferation. Cells from the spleen, bone marrow, and thymus of each of several strains of inbred mice, including xid CBA/N, poorly responsive to lipopolysaccharide, are stimulated by the mitogen, as are cells from BALB/c nude mice. The characteristics of the stimulatory effect are analogous in several important aspects to those of naturally occurring T cell-derived growth factors. In the absence of detectable numbers of T cells, both small and large B lymphocytes undergo mitosis in the presence of contaminated cell culture fluid, and B cells stimulated to divide by lipopolysaccharide are sustained for further rounds of replication by M. orale-containing cell culture fluid. The fluid also augments the stimulatory effect on thymocytes of suboptimum concentrations of phytohemagglutinin mimicking the effect of interleukin-1. Unlike with most naturally occurring lymphoid cell mitogens, however, the dividing cells do not go on to immunoglobulin secretion.