Failure of anti-Forssman antibodies to induce rejection of mouse heart xenografts

The Forssman antigen has been proposed to be a target for the xenograft rea ction in selected species combinations, including the rat and mouse, which are Forssman-negative and -positive species respectively. The mouse represe nts an important experimental model for a variety of immune-mediated diseas e processes, and the availability of a simple, inexpensive target antigen c ould provide an important tool for studying a selected portion of the immun ologic basis for the rejection of xenografts. We have examined the potentia l that antibodies directed against mouse Forssman antigen could cause the h yperacute rejection of mouse heart xenografts in naive rat recipients. The Forssman antibodies tested included rat anti-mouse (R-anti-M) antiserum, R- anti-M antiserum depleted of anti-Forssman (anti-F) antibodies, rat anti-sh eep red blood cell (SRBC) antiserum containing anti-F antibodies and a rat monoclonal anti-F IgM antibody. Our results demonstrate that the R-anti-M a ntiserum at day 4 post transplantation displayed significant titers (1:512- 4096) of hemagglutinating antibodies for SRBC and mild to moderate levels o f IgM that specifically binds to Forssman glycolipid (GalNAc alpha 1-3GalNA c beta 1-3Ga1 alpha 1-4Gal beta 1-4Glc beta 1-1 ceramide) as measured by an enzyme-linked immunosorbent assay (ELISA). Passive transfer of the R-anti- M serum to rats receiving mouse cardiac grafts immediately after transplant ation caused hyperacute rejection of the xenografts, Sequential immunoabsor ption of R-anti-M sera with SRBCs resulted in total removal of the anti-For ssman activity (as defined by negative hemagglutination titer and minimal b inding to Forssman glycolipid in ELISA). The anti-F Ab-depleted R-anti-M an tisera, however, retained the capacity to induce hyperacute rejection of th e mouse hearts [n = 6, median survival time (MST) 13 min] when passively tr ansferred to rat recipients. Anti-Forssman antibodies induced by immunizati on of LEW rats with SRBCs or a rat anti-Forssman monoclonal antibody, mAb M .1.22.25, exhibited substantial anti-Forssman activity (hemagglutinating ti ter 1:512-096 and moderate-to-strong binding to Forssman glycolipid in ELIS A respectively). These antibodies also failed, however, to trigger hyperacu te rejection of mouse cardiac xenografts. In conclusion, our results sugges t that the rat anti-Forssman antibodies, including those stimulated by mous e cardiac xenografts, do not appear to play a role in the immediate (hypera cute) rejection of mouse heart xenografts.