A Longitudinal Study of Humoral Immune Responses to SARS-CoV-2 Spike Proteins in Gamma-Interferon-inducible Lysosomal Thiol Reductase Deficient Mice

Abstract

Abstract The disulfide (S-S) bonds located in the receptor binding domain (RBD) of Spike (S) proteins of SARS-CoV-2 are critical for structural folding and viral entry. Once entered cells, S proteins, like other exogenous Ag, can be transported into endo/lysosomes, where gamma-interferon-inducible lysosomal thiol reductase (GILT) is the only thiol reductase located, which can reduce S-S bonds. GILT mediated S-S bond reduction in APC is critical in unfolding of native S-S bonds containing Ag, subsequently processing/presenting Ag, and, finally, triggering CD4 T cell responses. Recently, we also find GILT plays a role in Ab responses. Whether GILT mediates anti-SARSCoV-2 host defense responses remains unknown. We hypothesized that GILT is important in eliciting host humoral immune responses against SARS-CoV-2 by reducing S-S bonds and exposing Ag epitopes. To address this, we used 3 different types of S protein derived Ag to immunize GILT KO or B6 control mice and measured the specific Ab responses in a longitudinal study. GILT KO or B6 wildtype mice were given RBD, S1 region and the full length of extracellular domain of S proteins with CFA initially, and boosted at week 3 with the same Ag in IFA. Sera were collected at weeks 3 and 5, months 3, 4 and 5 for quantification of specific anti-RBD, S1 and S Ab. At all timepoints examined, GILT KO mice generated lower specific Ab titers to the immunogens, as compared to B6. Furthermore, decay of anti-specific Ab in GILT KO was faster than that in controls after immunization. Our preliminary findings showed an important role of GILT in humoral immune responses against SARS-CoV-2, suggesting unfolding S-S bonds is critical in this process. Recombinant GILT as a therapeutic might benefit patients with COVID-19. Supported by Seed Grant, Univ. of Utah