Surface Ig isotypes on cells responding to lipopolysaccharide by IgM and IgG secretion

Using the fluorescence-activated cell sorter (FACS), we have investigated that the Ig isotypes present on murine B cells, which can be polyclonally activated by lipopolysaccharide (LPS) in low cell density cultures. The LPS response was partly inhibited as a result of staining with anti-IgD and anti-IgM reagents, but not with anti-IgG reagents. The IgM ⁺, IgD ⁺, or IgG ^- fractionated cell populations gave both an IgM and an IgG response comparable to controls, whereas the response of the IgM ^-, IgD ^- cells was 5- to 20-fold lower. IgG ^- cells separated 1 day after LPS stimulation could still mount an IgM and IgG response indistinguishable from controls at the peak of the response. It is concluded that IgM ⁺, IgD ⁺, IgG ^- cells constitute the major LPS-sensitive cell population in the low cell density culture system and that IgG is not a necessary cell surface isotype for precursors of IgG-secreting cells.