Background: Indirect presentation of minor histocompatibility antigens (HA) as revealed by cross-priming of H2 heterozygous recipients effectively primes minor HA-specific cytolytic T lymphocytes (CTLs). However, it is not known if indirect priming generates CTLs specific for the same set of immunodominant minor HA recognized by CTLs primed by direct spleen cell injections. Methods: In order to indirectly prime minor HA-specific CTLs, we implanted (C57BL/6 x B6.C-H2(d))F1 recipients with BALB.B and BALB/c splenocytes loaded into immunoisolation devices that effectively preclude direct donorhost contact. Responder spleen cells from these recipients were stimulated in vitro to expand BALB.B- and BALB./c-specific CTLs to reveal classical cross-priming. Results: Tests of CTL specificity using (1) CXB recombinant inbred strain targets that express different arrays of BALB/c minor HA and (2) high-performance liquid chromatography fractions of peptides from BALB.B K(b) D(b) molecules revealed that anti-BALB.B CTLs were specific for two previously identified dominant peptides, CTT-2 and CTT-5, presented by K(b) molecules. Variation of responders and priming cells resulted in CTl responses to additional dominant peptides that had been identified previously with CTLs generated by direct priming with spleen cell injections. Indirect priming was not limited to this set of peptides recognized by CTLs in vitro because devices loaded with cells devoid of the CTL-detected peptides primed for accelerated skin allograft rejection. Conclusions: Indirect presentation of minor HA in vivo stimulates the generation of CTLs specific for a subset of dominant minor HA peptides recognized by CTLs primed by direct presentation.
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