Prominent IgM rheumatoid factor production by human cord blood lymphocytes stimulated in vitro with Staphylococcus aureus Cowan I

We previously reported that Staphylococcus aureus Cowan I (SAC) is a potent stimulant of IgM rheumatoid factor (RF) production by normal adult peripheral blood mononuclear cells. In the current study, we compared the capacity of normal adult peripheral blood mononuclear cells and cord blood mononuclear cells to produce IgM RF. Although both populations of cells consistently produced IgM RF in response to SAC, the quantity of RF produced by cord blood cells (128 +/- 18 ng/ml, mean +/- SEM) greatly exceeded that of adult cells (37 +/- 5 ng/ml, p less than 0.0005) even though both populations of cells demonstrated comparable total IgM responses. Remarkably, 16.7% of total IgM produced by cord blood cells in response to SAC showed RF activity compared to only 3.4% (p less than 0.0001) of the total IgM produced by SAC-stimulated adult cells. Thus, precursors of IgM RF-secreting cells are not only a consistent feature of the normal adult human B cell repertoire, but they are especially represented at the time of birth. These findings are consistent with the hypothesis that IgM RF originates from germ line genes and underscore the utility of SAC as a probe for analyzing the production of this autoantibody.