SARS-CoV-2 variants Omicron BA.4/5 and XBB.1.5 significantly escape T cell recognition in solid organ transplant recipients vaccinated against the ancestral strain

Abstract

Background: Immune-suppressed solid organ transplant recipients (SOTRs) display impaired humoral responses to COVID-19 vaccination, but T cell responses are incompletely understood. The highly infectious SARS-CoV-2 variants Omicron BA.4/5 and XBB.1.5 escape neutralization by antibodies induced by vaccination or infection with earlier strains, but T cell recognition of these lineages in SOTRs is unclear. Methods: We characterized Spike-specific T cell responses to ancestral SARS-CoV-2, Omicron BA.4/5 and XBB.1.5 peptides in a prospective study of kidney, lung and liver transplant recipients (n = 42) throughout a three- or four-dose ancestral Spike mRNA vaccination schedule. Using an optimized activation-induced marker assay, we quantified circulating Spike-specific CD4+ and CD8+ T cells based on antigen-stimulated expression of CD134, CD69, CD25, CD137 and/or CD107a. Results: Vaccination strongly induced SARS-CoV-2-specific T cells, including BA.4/5- and XBB.1.5-reactive T cells, which remained detectable over time and further increased following a fourth dose. However, responses to Omicron BA.4/5 and XBB.1.5 were significantly lower in magnitude compared to ancestral strain responses. Antigen-specific CD4+ T cell frequencies correlated with anti-receptor-binding domain (RBD) antibody titres, with post-second dose T cell responses predicting subsequent antibody responses. Patients receiving prednisone, lung transplant recipients and older adults displayed weaker responses. Conclusions: Ancestral strain vaccination stimulates BA.4/5 and XBB.1.5-cross-reactive T cells in SOTRs, but responses to these variants are diminished. Antigen-specific T cells can predict future antibody responses and identify vaccine responses in seronegative individuals. Our data support monitoring both humoral and cellular immunity in SOTRs to track effectiveness of COVID-19 vaccines against emerging variants.