Project Summary (2022-2027)

Suppression of humoral immunity by environmental contaminants that are prototypical aryl hydrocarbon receptor (AHR) ligands (e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)) have been demonstrated to suppress humoral immunity in laboratory animals and suggested by epidemiological evidence. The overall goal of this research is to define the molecular mechanism(s) responsible for AHR agonist-mediated suppression of antibody production by human primary B cells.

Prior studies comparing activated mouse, rat, and human primary B cells, RNAseq showed a small number of differentially expressed genes by TCDD. One gene that was only differentially regulated in human B cells compared to mouse and rat was the lymphocyte-specific protein tyrosine kinase (LCK). While LCK is expressed by T cells, subpopulations of B cells also express LCK, specifically CD5+ B cells, have shown that decreased IgM secretion by TCDD is due, in part, to selective effects on CD5+ B cells, termed innate-like B (ILB) cells. Although CD5+ ILBs constitute approximately 10-25% of circulating B cells and are the source for the majority of circulating IgM in the absence of an immune response. Further, CD5+ ILBs are the primary B cell type mediating humoral immunity early and late in life when the adaptive immune system is still developing or during late life stages when adaptive immunity is in decline. Protein analysis also showed induction by TCDD of LCK, the inhibitory receptor PD-1 and its ligand, PD-L2 on CD5+ ILBs. Induction of PD-1 and PD-L2 are especially important as they provide a mechanism for the suppression of the IgM response by AHR activation in CD5+ ILBs. LCK can phosphorylate the inhibitory region of the PD-1 receptor, facilitating immune suppression by PD-1. Inhibition of LCK activity protected CD5+ B cells from IgM suppression by TCDD, demonstrating LCK is critically involved.

Therefore, the hypothesis is: AHR-mediated suppression of IgM occurs through induction of the inhibitory receptor PD-1 and LCK, a kinase known to initiate PD-1- mediated immune regulation, preferentially on human primary CD5+ (innate-like) B cells. Aim 1 characterizes human CD5+ (innate-like) B cells as a highly sensitive population susceptible to suppression by AHR ligands. Aim 2 is to identify the role of induced checkpoint inhibitor, PD-1, and tyrosine kinase, LCK, in TCDD-mediated suppression of IgM production by CD5+ B cells.

Completion of the above aims has a strong potential to: (a) define B cell population(s) highly sensitive to impairment by AHR ligands critical for immunity against common pathogens; (b) identify a novel mechanism of immunotoxicity involving upregulation of the inhibitory receptor, PD-1; (c) characterize the role of LCK in IgM suppression by AHR activation using IFN? as a molecular probe; (d) utilize percent circulating CD5+ B cells as biomarker of sensitive populations to suppression of IgM by AHR ligands and (e) results for this experimental plan are being used to develop a calibrated TCDD PBPK model for risk assessment in collaboration with the Computational Modeling Core.