Unique processing pathways within recipient antigen-presenting cells determine IgG immunity against donor platelet MHC antigens

Recipient IgG immunity against leukoreduced donor platelets is dependent on indirect T-cell allorecognition and is suppressed in vivo by inhibitors (a minoguanidine, AMG) of inducible nitric oxide synthase (iNOS), To examine r ecipient processing pathways of donor platelet antigens, enriched macrophag es (antigen-presenting cells [APC]) from BALB/c (H-2(d)) mice were pulsed w ith allogeneic C57BL/6 (H-2(b)) platelets and transfused weekly into naive BALB/c mice. Platelet-pulsed APC stimulated IgG antidonor antibody producti on in 45% of recipients by the second transfusion and in 100% by the sixth transfusion; this response was enhanced by pulsing in the presence of inter feron-gamma. By the sixth transfusion, high-titer IgG1 (mean titer 4990) an d IgG2a (1933) isotypes specific for donor major histocompatibility complex (MHC) class I antigens were detected. Platelet pulsing in the presence of AMG or colchicine significantly inhibited the ability of APC to stimulate I gG alloantibodies; only 50% (P < .005) and 20% (P < .0001) of recipients, r espectively, produced antibodies by the sixth transfusion. AMG inhibition w as reversed by the addition of L-arginine, the substrate for iNOS, In contr ast, pulsing in the presence of chloroquine, the proteasome inhibitory pept ide MG115, or Brefeldin A enhanced APC immunity (70-100% of recipients anti body positive by the second transfusion [P < .05]); these agents allowed th e pulsed APC to stimulate IgG2a but inhibited IgG1 production and this corr elated with a reduction in serum interleukin (IL)-4 levels, The results sug gest that for donor platelet antigens to stimulate IgG alloantibodies, reci pient APC use the essential generation of nitric oxide and a noncytosolic, pH-independent processing pathway, which can be exploited as an effective i mmunotherapy target to further inhibit alloimmunization against leukoreduce d platelets, (C) 2000 by The American Society of Hematology.