Analysis of high and low responses toStaphylococcus aureus and interleukin 2 in human B lymphocytes

Fixed protein A-bearing staphylococci (SAC) stimulate human B cells via surface Ig, whereas IL-2 has been reported to provide a sufficient second signal for proliferation and differentiation. Using an ELISPOT assay to count cells secreting IgM, IgA, and IgG and flow cytometry with acridine orange to assess cell cycle progress, we have found that the purified B lymphocytes of a substantial minority (5/13) of healthy volunteers with normal serum Ig levels failed to differentiate to Ig secreting cells (ISC) in response to SAC + IL-2 (IgM, IgA, or IgG secreting cells, <5% of input B cells). High-responders generally formed 10–35% ISC. The proportions of B cells expressing IgG, IgA, IgM, or IgD were not different in the two groups. By average linkage cluster analysis, SAC/IL-2 high- and low-responders were shown to fall into two separate populations with respect to ISC. High- and low-responders tended to remain in the same group with repeated testing over several months, although some convergence was seen. The low-responders also showed significantly less advancement to late G1 and S phase than the high-responders, in the presence of SAC ± IL-2. Induction of IL-2 receptors on B cells by SAC + IL-2 was much greater in high-responders than in low-responders, as shown by flow cytometry with phycoerythrinconjugated IL-2. However, SAC + IL-2 induced transferrin receptors normally in low-responders, showing that some early activation steps occur in these cells. Low-responder B cells often improved their responses in the presence of macrophages and T cell supernatants. Finally, bypassing the surface Ig pathway using anti-CD3-activated T cells to stimulate B cells produced normal differentiation in low-responder B cells. Thus a subset of clinically normal individuals possesses B cells which fail to express IL-2 receptors, proliferate, and differentiate normallyin vitro in response to SAC + IL-2 yet can respond well to alternative activation pathways via T cells, monocytes, and their products.