Dipeptidyl peptidase I and granzyme A are coordinately expressed during CD8⁺ T cell development and differentiation

Dipeptidyl peptidase I (DPPI) is a granule protease that plays a requisite role in processing the proenzyme form of the CTL granule serine proteases (granzymes). This study assesses DPPI mRNA and enzyme expression during T lymphocyte ontogeny and CTL differentiation. The most immature CD3^- CD4^-CD8^- thymocytes were found to express >40-fold higher levels of DPPI mRNA, although levels of DPPI enzymatic activity in CD3^-CD4^-CD8^- thymocytes were only modestly higher than those seen for CD4⁺CD8⁺ or CD4⁺CD8^- thymocytes. More mature CD8⁺CD4^- thymocytes and CD8⁺ splenocytes expressed significantly higher levels of DPPI mRNA and enzymatic activity than CD4⁺CD8⁺ or CD4⁺CD8^- thymocytes. Granzyme A mRNA expression was observed in DPPI expressing CD3^-CD4^-CD8^- and CD8⁺CD4^- thymocytes and was also observed in CD8⁺CD4^- splenocytes; however, expression was not observed in CD4⁺CD8⁺ or CD4⁺CD8^- thymocytes. Both DPPI mRNA and granzyme A mRNA expression in CD8⁺ T cells decreased to very low or undetectable levels during the first 48 h after allostimulation in MLCs. However, peak levels of both DPPI and granzyme A expression were observed later in the course of CD8⁺ T cell responses to alloantigen, with DPPI mRNA expression peaking on either day 3 or day 4 and granzyme A expression peaking at the end of a 5-day MLR. These data indicate that DPPI is expressed at all stages of T cell ontogeny and differentiation in which granzyme A mRNA is detected; consequently, DPPI appears to be available for the processing and activation of granzyme A during both CD8⁺ T cell development and differentiation.