Non-depleting anti-CD4, but not anti-CD8, antibody induces long-term survival of xenogeneic and allogeneic hearts in alpha 1,3-galactosyltransferase knockout (GT-Ko) mice

The anti-galactose-alpha1,3-galactose (Gal) antibody (Ab) response followin g pig-to-human transplantation is vigorous and largely resistant to current ly available immunosuppression. The recent generation of GT-Ko mice provide s a unique opportunity to study the immunological basis of exnograft-elicit ed anti-Gal Ab response in vivo, and to test the efficacy of various strate gies at controlling this Ab response [1]. In this study, we compared the ab ility of non-depleting anti-CD4, and anti-CD8 to control rejection and anti body production in GT-Ko mice following xenograft and allograft transplanta tion. Hearts from baby Lewis rat or C3H mice were transplanted heterotopically in to GT-Ko. Non-depleting anti-CD4 (YTS177) and anti-CD8 (YTS105) Abs were us ed at 1 mg/mouse, and given as four doses daily from day -2 to 1 then q.o.d . till day 21. Xenograft rejection occurred at 3 to 5 days posttransplantat ion in untreated GT-Ko recipients, and was histologically characterized as vascular rejection. Anti-CD4, but not anti-CD8, Ab treatment prolonged xeno graft survival to 68 to 74 days and inhibited anti-Gal Ab as well as xeno-A b production. In four of the five hearts from anti-CD4 mAbs-treated GT-Ko m ice, we observed classic signs of chronic rejection, namely, thickened inti ma in the lumen of vessels, significant IgM deposition, fibrosis and modest mononuclear cell infiltrate of Mac-1(+) macrophages and scattered T cells (CD8>CD4). Xenograft rejection in untreated, as well as anti-CD4- and anti- CD8-treated, recipients was associated with increased intragraft IL-6, IFN- gamma and IL-10 mRNA. C3H allografts were rejected in 7 to 9 days by untreated GT-Ko mice and wer e histologically characterized as cellular rejection. Treatment with anti-C D4 and anti-CD8 mAb resulted in graft survivals of >94.8 and 11.8 days, res pectively. Anti-CD4 mAb treatment resulted in a transient inhibition of all oreactive and anti-Gal Ab production. The presence of circulating alloreact ive and anti-Gal Abs at >50 days post-transplant was associated with signif icant IgM and IgG deposition in the graft. Yet, in the anti-CD4, mAb-treate d group, the allografts showed no signs of rejection at the time of sacrifi ce (>100 days posttransplantation). All rejected allografts had elevated le vels of intragraft IL-6, IFN-gamma and IL-10 mRNA, while the long-surviving anti-CD4-treated allografts had reduced mRNA levels of these cytokines. Collectively, our studies suggest that the elicited xeno-antibody productio n and anti-Gal Ab production in GT-Ko mice are CD4(+) T-cell dependent. The majority of xenografts succumbed to chronic rejection, while allografts su rvived with minimal histological change, despite elevated levels of circula ting alloAbs. Thus, immunosuppression with anti-CD4 mAb therapy induces lon gterm survival of allografts more effectively than to xenografts.