Soluble FcR Block Suppressor T Cell Activity at Low Concentrationin VitroAllowing Isotype-Specific Antibody Production

IgA and IgG binding factors (BF) can be found in the supernatant (Thsup) of cultures containing macrophages and CD4+T cells stimulated with particulate antigens such as SRBC. Previous work indicated that these IgBF, when mixed with normal serum immunoglobulin, could block the activity of suppressor T cells (Ts) and allow IgA and IgG PFC responsesin vitro.We present serologic and functional evidence that IgABF and IgGBF in Thsup are soluble FcαR and FcγRII(or III), respectively. Thsup adsorbed on affinity columns containing anti-FcγRII/III mAb or murine IgG failed to augment IgG PFC responses. Material eluted from either the IgG or anti-FcγRII/III columns could be added back, interchangeably, to the adsorbed Thsup and restore IgG PFC. Recombinant murine FcγRII (rFcγRII), added to the same adsorbed Thsup at 0.01 to 0.5 ng/ml, resulted in a similar augmentation of IgG PFC. Interestingly, much higher concentrations of rFcγRII (10–100 ng/ml) could not augment IgG PFC responses. Protein dot blots showed that Thsup and the eluted material from murine IgG columns contained structures reactive with the FcγRII/III mAb. Similar studies using purified FcαR revealed that IgABF eluted from IgA or anti-FcαR columns was in fact FcαR. Cross-adsorption studies indicated clearly that the IgGBF (FcγRII/III) and the IgABF (FcαR) were separate molecules produced in the same Thsup and that each regulated their respective Ig isotype independently. Thus, cultures of splenic macrophage and CD4+T cells, in the presence of particulate antigens such as SRBC, generate both FcγRII/III and FcαR. This soluble FcR in combination with serum Ig act to block isotype-specific Tscells at low concentrationin vitro.