In FY 21 we continued to focus the immunotoxicology research program on in vitro approaches. A major effort involved investigating how interindividual susceptibility factors and environmental risk factors impact the response to viral infection, using an in vitro human whole blood culture system. Endpoints include lymphocytotoxicity, cytokine release, and Natural Killer cell activity. We have screened >80 individual samples to date and are evaluating how intrinsic factors such as age, gender and ethnicity influence the response to influenza and SARS-CoV-2 antigens. An additional aspect of the project will use the same culture system and endpoints to investigate the applicability of the test system to investigate the immunotoxicity of polycyclic aromatic compounds and how exposure to these environmental agents may affect susceptibility to viral infection. These data will be referenced against mouse in vivo data previously collected in the PAC-MAP mixtures studies and serve to anchor human relevance and strengthen the data that can be generated using the in vitro system. BRT is currently working on development of additional in vitro tools for this culture system that will facilitate interrogation of humoral mediated immunity and T-cell driven cell-mediated immunity. This in vitro toolbox will be critically important to identify chemicals that have the potential to modulate immune function in humans, and will position NTP at the cutting edge of testing in this field.