: Smoking causes widespread epigenetic changes that have been linked with an increased risk of smoking-associated diseases and elevated mortality. Of particular interest are changes in the level of T cells expressing G-protein-coupled receptor 15 (GPR15), a chemokine receptor linked with multiple autoimmune diseases, including inflammatory bowel disease, multiple sclerosis and psoriasis. Accordingly, a better understanding of the mechanisms by which smoking influences variation in the GPR15 helper T cell subpopulation is of potential interest. : In the current study, we used flow cytometry and digital PCR assays to measure the GPR15CD3CD4 populations in peripheral blood from a cohort of = 62 primarily African American young adults (aged 27-35 years) with a high rate of tobacco and cannabis use. We demonstrated that self-reported tobacco and cannabis smoking predict GPR15CD3CD4 helper T cell levels using linear regression models. Further, we demonstrated that methylation of two candidate CpGs, cg19859270, located in , and cg05575921, located in the gene (), were both significant predictors of GPR15CD3CD4 cell levels, mediating the relationship between smoking habits and increases in GPR15CD3CD4 cells. As hypothesized, the interaction between cg05575921 and cg19859270 was also significant, indicating that low cg05575921 methylation was more strongly predictive of GPR15CD3CD4 cell levels for those who also had lower cg19859270 methylation. : Smoking leads changes in two CpGs, cg05575921 and cg19859270, that mediate 38.5% of the relationship between tobacco and cannabis smoking and increased GPR15 T levels in this sample. The impact of cg19859270 in amplifying the association between cg05575921 and increased GPR15 T levels is of potential theoretical interest given the possibility that it reflects a permissive interaction between different parts of the adaptive immune system.