Stimulation of germinal center B lymphocyte proliferation by an FDC-like cell line, HK.

To identify the signals given to the germinal center (GC) B cells by FDC, we have established an FDC-like cell line, HK. HK cells have the important functions of FDC, which are the preferential binding and stimulation of GC B cells. Cell-cell interaction between IgD- B cells and HK cells resulted in the rescue of these cells from apoptosis, whereas the majority of unbound B cells or B cells cultured in the absence of HK cells underwent apoptosis. We investigated the effects of HK cells on the subpopulations of tonsillar B lymphocytes that are at different stages of maturation and differentiation. The subpopulations of tonsillar B cells were purified by panning and/or MACS according to the surface expression of IgD, CD38, and CD44. Although HK cells alone did not stimulate B cell proliferation, HK cells in the presence of either anti-mu or anti-CD40 significantly enhanced the proliferation of B cells. The data show that HK cells preferentially bind, rescue, and stimulate GC B cells (IgD-CD38+) generating CD38-CD44+ memory B cells. The costimulation activity of HK cells is also provided in a form of soluble factor(s). Hence, the phenotypic and functional analysis of HK cells suggests that these cells may be derived from FDC.