Modulation of anti-sheep erythrocyte primary antibody response by protein A in normal mice

Protein A (PA), a cell wall glycoprotein of Staphylococcus aureus Cowan I, is known to possess immunomodulatory and antitumor activities. In the present study the effect of protein A on the anti-sheep erythrocyte primary antibody response in normal mice has been investigated. Intraperitoneal administration of PA resulted in suppression of primary antibody response in mice immunized with sheep red blood cells (SRBC) as assessed by plaque forming cell (PFC) assay. The suppressive effect was dose dependent. Maximum suppression in PFC response was observed at 12 micrograms PA/animal and could be observed as low as 1 microgram PA/animal. The suppression at 6 and 12 micrograms PA/animal was significant when compared with control values (p < 0.02 and p < 0.001, respectively). However, no significant suppression was recorded at 1 microgram dose. The suppression was proportional to the numbers of administrations of PA to the mice and critically dependent on the timing of inoculation of PA. PA produced pronounced suppression when injected prior to the immunization of animals with SRBC. PA reduced splenic localization of 51Cr labelled SRBC to 55%. It is proposed that the altered localization of antigen (SRBC) in spleen may be responsible for reduced plaque forming cells in normal mice.