Simultaneous Reconstitution of Multiple CMV-Specific CD8⁺ T-Cell Populations with Divergent Functionality in Hematopoietic Stem Cell Transplant Recipients.

Recent studies using direct stimulation of PBMC from CMV-positive individuals with optimal CTL epitope peptides have identified new antigens that are targets of the host immune response. The growing number of targets of the cellular immune response has prompted an evaluation of which antigens are potentially associated with protective immunity. A panel of seven human cytomegalovirus (CMV) epitope peptides and the corresponding MHC-I tetramers was used to evaluate cellular immunity in six healthy seropositive donors and in six hematopoietic stem cell transplant recipients (HSCT). Broad CMV-specific responses were found to epitopes within several CMV polypeptides that were restricted by multiple HLA alleles in several individuals. The results document the simultaneous expansion of several of these CD8⁺ T-lymphocyte populations following allogeneic HSCT. The combined levels of CMV epitope-specific T-cells exceeded 20% of CD8⁺ T-lymphocytes in some individuals. The cytotoxic functionality of 26 different populations of CMV-specific T-lymphocytes detected within this group of 12 individuals was addressed by utilizing a recently described assay that measures transient surface levels of the lysosomal membrane proteins LAMP-1 (CD107a) and LAMP 2 (CD107b) after peptide stimulation. This degranulation/mobilization assay can be combined with tetramer staining of antigen-specific CD8⁺ T-lymphocytes, and has potential as a surrogate marker for cytotoxic function. We found that a significant proportion of CD8⁺ T-lymphocytes specific for epitopes within the CMV pp65 and pp50 gene products had functional potential as measured by this assay (median percentage of cells within 14 T-cell populations staining with pp65 or pp50 tetramers that degranulated on stimulation with cognate peptide = 26.0% and 19.8%). By contrast, CD8⁺ T-lymphocytes specific for epitopes within the CMV IE-1 gene product had markedly reduced functionality (median percentage of cells within 12 T-cell populations staining with IE-1 tetramers that degranulated on stimulation with cognate peptide = 5.6%). This difference was significant (p= 0.003 by an F-test after adjusting for HLA and using interaction of subjects and epitopes as the error). This reduced degranulation efficiency of IE-1-specific T cells is consistent with their inefficient cytotoxic recognition of CMV-infected autologous fibroblasts. Further characterization of this functional dichotomy includes comparison of cell surface marker phenotype and cytokine release in response to antigenic presentation. These functional differences between T-lymphocyte populations within the same individual have implications for choosing antigens that are both protective and necessary to include in a CMV vaccine for transplant patients at risk for infectious complications after viral reactivation.