| Summary: | Background and Aim: Immune status profile can predict response to vaccination, while lymphocyte phenotypic alterations represent its effectiveness. We prospectively evaluated these parameters in kidney transplant recipients (KTRs) regarding Tozinameran (BNT162b2) vaccination. Method: In this prospective monocenter observational study, 39 adult KTRs, on stable immunosuppression, naïve to COVID-19, with no protective humoral response after two Tozinameran doses, received the third vaccination dose, and, based on their immunity activation, they were classified as responders or non-responders. Humoral and cellular immunities were assessed at predefined time points (T<sub>0</sub>: 48 h before the first, T<sub>1</sub>: 48 h prior to the third and T<sub>2</sub>: three weeks after the third dose). Results: Responders, compared to non-responders, had a higher total and transitional B-lymphocyte count at baseline (96.5 (93) vs. 51 (52)cells/μL, <i>p</i>: 0.045 and 9 (17) vs. 1 (2)cells/μL, <i>p</i>: 0.031, respectively). In the responder group, there was a significant increase, from T<sub>0</sub> to T<sub>1,</sub> in the concentrations of activated CD4+ (from 6.5 (4) to 10.08 (11)cells/μL, <i>p:</i> 0.001) and CD8+ (from 8 (19) to 14.76 (16)cells/μL, <i>p</i>: 0.004) and a drop in CD3+PD1+ T-cells (from 130 (121) to 30.44 (25)cells/μL, <i>p</i>: 0.001), while naïve and transitional B-cells increased from T<sub>1</sub> to T<sub>2</sub> (from 57.55 (66) to 1149.3 (680)cells/μL, <i>p</i> < 0.001 and from 1.4 (3) to 17.5 (21)cells/μL, <i>p</i>: 0.003). The percentages of memory and marginal zone B-lymphocytes, and activated CD4+, CD8+ and natural killer (NK) T-cells significantly increased, while those of naïve B-cells and CD3+PD1+ T-cells reduced from T<sub>0</sub> to T<sub>1</sub>. Conclusions: Responders and non-responders to the third BNT162b2 dose demonstrated distinct initial immune cell profiles and changes in cellular subpopulation composition following vaccination.
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