The effect of xenoreactive antibody and B cell depletion on hyperacute rejection of guinea pig-to-rat cardiac xenografts

Xenotransplantation between phylogenetically distant species is prevented by hyper acute rejection (HAR), a process that is thought to be initiated by the binding of naturally occurring xenoreactive antibodies (NAb) to the endothelium of the xenograft (Xg) with subsequent activation of the classical pathway of C. The relative role of direct alternative pathway C activation in HAR is controversial. To evaluate the role of NAb in HAR of discordant rodent Xg, LEW rats were treated from the day of birth with i.p. injections of rabbit anti-rat IgM antiserum (RARM), or with mAb specific for rat k-light chain (OX12) or rat class II MHC (14–4–4S, Y-3P, or 10–2.16), in an effort to deplete B cells and NAb. These rats then underwent xenotransplantation with discordant guinea pig hearts. RARM was effective in depleting rats (n=5) of B cells, serum IgM, and rat NAb directed against guinea pig cells, but guinea pig cardiac Xg survival was not prolonged compared with PBS-treated controls (n=5), possibly due to the rabbit NAb specific for guinea pig cardiac tissue that were passively transferred in the RARM preparation. Of the anti-B cell mAb used to avoid this passive transfer of NAb, mAb 14–4–4S was highly effective (n=9) in depleting the peripheral blood and spleen of B cells and the serum of IgM and NAb. Guinea pig cardiac Xg survival, however, was again not prolonged (n=5), and rejected Xg from the B cell-and NAb-de-pleted recipients demonstrated rat C3 deposition in the absence of rat IgM and IgG. This study demonstrates that while neonatal anti-B cell antibody treatment can effectively deplete B cells and NAb in the rat, such treatment does not significantly prolong cardiac Xg survival in this well-established guinea pig to rat xenotransplantation model. These findings suggest that in addition to NAb depletion, inhibition of alternative C pathway activation and other humoral mechanisms may be necessary to prevent HAR and allow successful xenotransplantation.