Lomash, V.; Srinivasan, M.; Pitthala, M.; Sayeed, A.; Venkatesan, G.; Joseph, B.

Evaluation of unintended immunotoxicity represents an important component of nonclinical safety assessment as perturbation of immune function may increase susceptibility to infection impair vaccine responses, and disrupt immune homeostasis. Regulatory guidance, including the ICH S8 Immunotoxicity Guideline recommends a weight of evidence approach in which observations from conventional toxicological endpoints are integrated with functional immune assays to support interpretation of immune system effects. The present study applied an integrated immunotoxicity evaluation framework to examine concordance among structural functional and cellular immune endpoints in male Sprague Dawley rats using a well characterized immunosuppressive reference compound. Hematological evaluation revealed leukopenia characterized primarily by lymphocyte depletion. Reductions in spleen and thymus weights were accompanied by histopathological evidence of lymphoid depletion in multiple immune tissues including spleen thymus lymph nodes Peyers patches, and bone marrow. Functional immune competence was assessed through hemagglutination antibody response to sheep red blood cells and delayed-type hypersensitivity assays, both of which demonstrated marked suppression of adaptive immune responses. Flow cytometric immunophenotyping further demonstrated substantial reductions in B-cell populations and decreases in CD4&(plus) and CD8&(plus) T-cell counts, whereas NK cell populations were comparatively less affected. The concordance of hematological alterations, lymphoid tissue changes, impaired functional immune responses, and lymphocyte subset depletion provides integrated evidence of immune system perturbation. These findings demonstrate that complementary immunotoxicity endpoints collectively support hazard characterization of immune system effects under GLP conditions.