Expanded CD16+CD56+Granzyme B+ NK like CD8+ T cells an off target effect of bruton's tyrosine kinase inhibitors in Waldenström macroglobulinemia.

Bruton's tyrosine kinase inhibitors (BTKi), widely used in Waldenström macroglobulinemia (WM), are known to impair B cell function, but their broader immunological effects remain unclear. We investigated the T cell compartment in patients with WM receiving BTKi, including those who underwent a supervised treatment pause before a third COVID-19 mRNA vaccine dose, as well as treatment naïve controls. Using flow cytometry, antigen-specific T cell assays, and single-cell RNA sequencing, we found that BTKi-treated patients exhibited a reduction in naïve T cells, enrichment of terminal effector CD8⁺ Temra cells, and lower PD-1 expression. Despite poor vaccine-induced RBD-specific memory B cells, CD4⁺ and CD8⁺ T cell responses to SARS-CoV-2 remained detectable, with enhanced CD8⁺ responses to the omicron variant in BTKi-treated individuals. Single-cell transcriptomics revealed an expansion of CD16⁺ CD56⁺ Granzyme B⁺ NK-like CD8⁺ T cells in BTKi-treated patients, which further increased following BTKi interruption. These cells expressed a potent cytotoxic and NK-like transcriptional phenotype. Our findings identify a novel off-target effect of BTKi therapy: expansion of cytotoxic NK-like CD8⁺ T cells, with implications for immune monitoring and vaccine responsiveness in BTKi-treated patients.