Regulation of the antibody response by the acute phase reactant: mouse serum amyloid P-component (SAP)

Serum amyloid P-component (SAP) is the major acute phase reactant (APR) of mice. Purified mouse SAP at 0.1 to 10.0 micrograms/ml selectively suppressed the secondary in vitro IgG antibody plaque-forming cell (PFC) response to the T-dependent antigen TNP-KLH but not to the T-independent antigens TNP-LPS and DNP-Lys-Ficoll. The suppression was antigen nonspecific. The mechanism of suppression occurred primarily through the activation of Lyt-1+, I-J+ suppressor-inducer cells, which in turn activated a Lyt-2+ suppressor T-cell population. The activity of preexisting, antigen-specific Lyt-2+ suppressor T cells was not influenced by SAP. The antigen-nonspecific suppressor T cells generated by SAP were sensitive to cyclophosphamide. Removal of SAP from the culture fluid with rabbit anti-Mo SAP antibody or agarose beads abrogated the suppression. Pentraxin proteins closely related to mouse SAP, such as human SAP and hamster female protein (FP), also displayed immunoregulatory activity of the antibody response by the same cellular mechanism. The results suggest that SAP regulates antibody responses by the activation of suppressor-inducer T cells and that the regulation of the antibody response during the acute stage of inflammation may occur via SAP.