Development of a BiTE®-mediated CD8+ cytotoxic T-lymphocyte activity assay to assess immunomodulatory potential of drug candidates in Cynomolgus macaque.

The immunotoxic potential of drug candidates is assessed through the examination of results from a variety of studies and endpoints. While the functional assessment of CD8⁺ cytotoxic T-lymphocytes (CTL) is well-characterized in the clinic, the lack of a robust macaque CTL functional assay has been an important hurdle in evaluating and accurately quantifying cell-mediated CD8⁺ T-cell effector responses in the nonclinical setting. This paper describes the development of an assay to measure CTL activity in peripheral blood mononuclear cells (PBMC) isolated from Cynomolgus macaques. A human EGFR/CD3 Bispecific T-cell Engager (BiTE^®) was used to mount a robust CD8⁺ T-cell response in the presence of target-expressing cells. Upon target engagement, degranulation of CD107a and production of interferon (IFN)-γ both reliably indicated a robust functional response in CD8⁺ T-cells. The BiTE^®-mediated stimulation method proved to be favorable when compared to other methods of stimulation in the absence of target cells. These studies demonstrated acceptable longitudinal variability of the functional assay and sensitivity to dexamethasone-mediated immunosuppression. Taken together, the results indicated an assay leveraging CD3-bispecific antibodies and target-expressing cells can provide a robust approach to the in vitro or ex vivo assessment of CTL function in Cynomolgus macaques. Because the impairment of CTL activity by immunomodulators is recognized to be an important contributor to decreased antiviral defense and increased carcinogenicity risk, we believe that this novel assay to be a valuable addition to the immunotoxicology assessment of therapeutic drug candidates.