Indirect allorecognition of HLA class I peptides by CD4(+) cytolytic T lymphocytes

T-cell responses co alloantigens can occur either by ''direct'' recognition of donor MHC molecules, or ''indirect'' recognition of MHC peptides in association with self-MHC. To evaluate human T cells mediating indirect allorecognition, a CD4(+) TCL and clones specific for HLA-Al or HLA-B8 (residues 60-84) were generated from normal PBLs (A2,29 B62,- DR1,4 DQ3). Most clones were Al specific (16 out of 17 tested), HLA-DR4 restricted (8 out of 8), and lysed targets pulsed with Al peptide (16 out of 16). An amino acid substitution at position 86 of the DR4 beta chain (G --> V) abrogated the capacity of CD4(+) CTLs to lyse target cells. Chloroquine treatment of Al-pulsed targets reduced their susceptibility to lysis, indicating a requirement for peptide processing. The TCL and clones were stimulated to proliferate by cells bearing intact HLA-Al when autologous APCs were present, indicating chat the epitope contained within the Al 60-84 peptide being recognized is produced when APCs process native HLA-Al. Furthermore, the clones and TCL did not recognize HLA-Al on target cells carrying this allele plus self-HLA-DR4. These studies suggest a much wider role for CD4(+) T cells in allograft immunity.