Extreme leukoreduction of major histocompatibility complex class II positive B cells enhances allogeneic platelet immunity

In a murine model of platelet alloimmunization. we examined the definitive role that mononuclear cells (MC) have in modulating platelet immunity by using platelets from severe combined immunodeficient (SCID) mice. CB.17 (H-2(d)) solo or BALB/c (H-2d) mouse platelets were transfused weekly into fully allogeneic CBA (H-2(k)) mice and antidonor antibodies measured by flow cytometry. Me levels in BALB/c platelets were 1.1 +/- 0.6/mu L and SCID mouse platelets could be prepared to have significantly lower (<0.05/mu L) MC numbers. Transfusions with 10(8) BALB/c platelets (containing approximate to 100 MC/transfusion) stimulated IgG antidonor antibodies in 100% of the recipients by the fifth transfusion, whereas 108 SCID mouse platelets (containing approximate to 5 MC/transfusion) stimulated higher-titered IgG alloantibodies by the second transfusion. When titrations of BALB/c peripheral blood Me were added to the SCID mouse platelets, levels approaching 1 MC/mu L reduced SCID platelet immunity to levels similar to BALB/c platelets. Characterization of the alloantibodies showed that the low levels of MC significantly influenced the isotype of the antidonor IgG; the presence of 1 MC/mu L was associated with induction of noncomplement fixing IgG1 antidonor antibodies, whereas platelet transfusions, devoid of MC (<0.05/mu L). were responsible for complement-fixing IgG2a production. When magnetically sorted defined subpopulations of MC were added to the SCID platelets, major histocompatability complex (MHC) class II positive populations, particularly B cells, were found to be primarily responsible for the reduced solo mouse platelet immunity. The presence of low numbers of MC within the platelets was also associated with an age-dependent reduction in platelet immunogenicity; this relationship however, was not observed with SCID mouse platelets devoid of MC. The results suggest that a residual number of MHC class II positive B cells within allogeneic platelets are required for maximally reducing alloimmunization. (C) 1999 by The American Society of Hematology.