Different requirements of ICAM-1/LFA-1 adhesion in allorecognition and self-restricted antigen recognition by class II-specific T cell clones.

We have analyzed the influence of non-antigen-specific interactions between ICAM-1 and LFA-1 in target recognition by allospecific and antigen-specific T cells at the clonal level, using human and mouse fibroblasts transfected with HLA-DR1 or DR2 with or without co-expression of ICAM-1, as antigen-presenting cells. The results show a great heterogeneity in the requirements for ICAM-1/LFA-1 interactions for antigen-specific and alloreactive T cell responses and this requirement may depend on the avidity of any particular interaction. The data also show that for most alloreactive clones, ICAM-1/LFA-1 adhesion is not sufficient to facilitate efficient T cell recognition of its target molecule. HLA class II recognition by a large proportion of the DR1- and DR2-specific alloreactive clones studied was different for class II molecules expressed on murine or human fibroblasts compared to human lymphoid cells, and was independent of ICAM-1 expression on the stimulator cells. The inability of some T cell clones to recognize HLA-class II expressed on non-lymphoid cells suggests the absence of specific epitopes and could be due to the lack of the relevant peptides, either because they are derived from species-specific proteins or to differences in processing of endogenous antigen in the transfected stimulator cells.