Incorporation of antigen 125I IgG into particular cell compartments: binding by chromatin

Incorporation of [125I]IgG into spleen cells was studied in vivo and in vitro. In vivo, the antigen after uptake into the cytoplasm migrated into cell nuclei, where it was bound to chromatin up to the saturation level. One day after immunization the constant level of [125I]IgG was 1.3 X 10(12) molecules per spleen (10(8) cells). The same number of [125I]IgG molecules were bound to chromatin in cell cultures. The uptake of [125I]IgG was competitively inhibited by non-labelled IgG. Binding of [125I]IgG molecules reextracted from cytoplasm and chromatin with specific anti-human IgG serum argues against the uptake of degraded [125I]IgG molecules. [125I]IgG was tightly bound to DNA. Approximately 50 per cent of [125I]IgG was present in the residual chromatin fraction (after removal of 0.35 M and 2 M NaCl-soluble fractions) and 40 per cent was complexed with DNA (after removal of histones and non-histones AP1, AP2, AP3 and AP4). Binding of [125I]IgG by isolated chromatin was inhibited by the cytoplasmic fraction but not by BSA. Binding of [125I]IgG by fractionated chromatin, (chromatins remaining after removal of 0.35M, and 2M NaCl-soluble fractions or histones + non-histones AP1 + AP2 + AP3 + AP4) occurred at a level similar to that observed with native chromatin. The results suggest that interaction of antigen with immunocompetent cells is not restricted to the cell surface but that antigen seems to be taken up into cytoplasm, migrates to the nuclei and is bound to chromatin, probably directly to DNA. The results are discussed in relation to the induction of the immune reaction.