THE HUMAN CYTOTOXIC T-LYMPHOCYTE (CTL) RESPONSE TO CYTOMEGALOVIRUS ISDOMINATED BY STRUCTURAL PROTEIN PP65 - FREQUENCY, SPECIFICITY, AND T-CELL RECEPTOR USAGE OF PP65-SPECIFIC CTL

Cytotoxic T lymphocytes (CTL) appear to play an important role in the control of human cytomegalovirus (HCMV) in the normal virus carrier: p revious studies have identified peripheral blood CD8(+) CTL specific f or the HCMV major immediate-early gene product (IE1) and more recently , by bulk culture and cloning techniques, have identified CTL specific for a structural gene product, the lower matrix protein pp65, In orde r to determine the relative contributions of CTL which recognize the H CMV proteins IE1, pp65, and glycoprotein B (gB) to the total HCMV-spec ific CTL response, we have used a limiting-dilution analysis system to quantify HCMV-specific CTL precursors with different specificities, a llowing the antigenic specificity of multiple shortterm CTL clones to be assessed, in a group of six healthy seropositive donors. All donors showed high frequencies of HCMV-specific major histocompatibility com plex-restricted CTL precursors. There was a very high frequency of CTL specific for pp65 (lower matrix protein); IE1-specific CTL were also detectable at lower frequencies in three of five donors, while CTL dir ected to gB were undetectable. A pp65 gene deletion mutant of HCMV was then used to estimate the contribution of pp65-specific CTL to the to tal HCMV-specific CTL response; this showed that between 70 and 90% of all CTL recognizing HCMV-infected cells were pp65 specific. Analysis of the peptide specificity of pp65-specific CTL showed that some donor s have a highly focused response recognizing a single peptide; the T-c ell receptor V beta gene usage in these two donors was shown to be rem arkably restricted, with over half of the responding CD8(+) T cells ut ilizing a single V beta gene rearrangement. Other subjects recognized multiple pp65 peptides: nine new pp65 CTL peptide epitopes were define d, and for five of these the HLA-presenting allele has been identified . All four of the HLA A2 donors tested in this study recognized the sa me peptide. This apparent domination of the CTL response to HCMV durin g persistent infection by a single structural protein, irrespective of major histocompatibility complex haplotype, is not clearly described for other persistent virus infections, and the mechanism requires furt her investigation.