Age-Related Persistent Clonal Expansions of CD28−Cells: Phenotypic and Molecular TCR Analysis Reveals both CD4+and CD4+CD8+Cells with Identical CDR3 Sequences

In a small group of subjects we had identified persistent expansions (range 6–72%) of CD4⁺CD8⁺double-positive (DP) peripheral blood (PB) cells which express the CD8 α/α homodimer. Here, DP cells present in a larger cohort were further investigated and found by FACS analysis to express a single or a dominant TCRBV family. In these subjects, with a mean age of about 64 years, expansions of CD4⁺cells with the same TCRBV family specificity as in the respective DP cells also were consistently detected. TCR heterogeneity of the dominant TCRBV family was specifically evaluated: The amplified CDR3 region was cloned and found to consist of one single or two largely dominant sequence patterns. Furthermore, cloning of the CDR3 region from FACS-sorted DP, CD4⁺, or CD8⁺cells indicates that both DP and CD4⁺, but not CD8⁺cells, isolated from the same individual possess a striking identity of the CDR3 regions. As indicated by FACS analysis, the clonally expanded cells occur in the CD4⁺CD28⁻cells. Taken together, these results suggest that expanded CD4⁺CD28⁻cells might also acquire CD8 α/α expression and become DP and imply that CD4 clonality is a more frequent phenomenon than previously suspected. In conclusion, the persistent expansions described in this report represent a novel group of age-related benign clonal expansions of still undefined significance of a rare CD28⁻T cell subset.