Transplant recipients, who receive therapeutic immunosuppression to prevent graft rejection, are characterized by high coronavirus disease 2019 (COVID-19)–related mortality and defective response to vaccines. We observed that previous infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but not the standard two-dose regimen of vaccination, provided protection against symptomatic COVID-19 in kidney transplant recipients. We therefore compared the cellular and humoral immune responses of these two groups of patients. Neutralizing anti–receptor-binding domain (RBD) immunoglobulin G (IgG) antibodies were identified as the primary correlate of protection for transplant recipients. Analysis of virus-specific B and T cell responses suggested that the generation of neutralizing anti-RBD IgG may have depended on cognate T-B cell interactions that took place in germinal center, potentially acting as a limiting checkpoint. High-dose mycophenolate mofetil, an immunosuppressive drug, was associated with fewer antigen-specific B and T follicular helper (T _FH ) cells after vaccination; this was not observed in patients recently infected with SARS-CoV-2. Last, we observed that, in two independent prospective cohorts, administration of a third dose of SARS-CoV-2 mRNA vaccine restored neutralizing titers of anti-RBD IgG in about 40% of individuals who had not previously responded to two doses of vaccine. Together, these findings suggest that a third dose of SARS-CoV-2 mRNA vaccine improves the RBD-specific responses of transplant patients treated with immunosuppressive drugs.

The diversity of vaccination modalities and infection history are both variables that have an impact on the immune memory of individuals vaccinated against SARS-CoV-2. To gain more accurate knowledge of how these parameters imprint on immune memory, we conducted a long-term follow-up of SARS-CoV-2 spike protein–specific immune memory in unvaccinated and vaccinated COVID-19 convalescent individuals as well as in infection-naïve vaccinated individuals. Here, we report that individuals from the convalescent vaccinated (hybrid immunity) group have the highest concentrations of spike protein–specific antibodies at 6 months after vaccination. As compared with infection-naïve vaccinated individuals, they also display increased frequencies of an atypical mucosa-targeted memory B cell subset. These individuals also exhibited enhanced T _H 1 polarization of their SARS-CoV-2 spike protein–specific follicular T helper cell pool. Together, our data suggest that prior SARS-CoV-2 infection increases the titers of SARS-CoV-2 spike protein–specific antibody responses elicited by subsequent vaccination and induces modifications in the composition of the spike protein–specific memory B cell pool that are compatible with enhanced functional protection at mucosal sites.

Advanced chronic kidney disease (CKD) is associated with impaired humoral immunity, contributing to increased infection-related mortality and suboptimal vaccine responses, as notably observed during the COVID-19 pandemic. CKD is also marked by the accumulation of uremic toxins, but whether they directly influence T and B cell functionality remains unclear. In this translational study, we integrated clinical and biological data from 106 CKD patients with mechanistic insights from in vitro and in vivo murine models to identify the mechanisms underlying CKD-associated defects in humoral responses against T cell-dependent antigens. Contrary to our initial hypothesis, indoxyl sulfate—despite its known ability to activate Aryl hydrocarbon Receptor signaling in monocytes—did not directly impair T–B cell cooperation in coculture assays. Similarly, plasma levels of ten major uremic toxins showed no correlations with vaccine-induced antibody titers in patients. Instead, systemic inflammation emerged as the primary driver of defective humoral immunity. Murine models further confirmed that inflammation, rather than uremia alone, induces lymphopenia, disrupts lymphoid architecture, and ultimately impairs antibody production. These findings indicate that CKD-associated inflammation, rather than a direct effect of uremic toxins on adaptive immune effectors, underlies humoral immune dysfunction in CKD. Targeting inflammation may, therefore, offer a promising strategy to improve vaccine efficacy and reduce infection-related complications in this vulnerable population.

Maxime Espi, Christophe O Soulage, Laetitia Koppe; Should we integrate the gut microbiota composition to manage idiopathic nephrotic syndrome?, Nephrology Dialy