Distinct T and innate-like lymphocyte reprogramming following lyophilized fecal microbiota transplantation in recurrent <i>C. difficile</i> infection

Fecal microbiota transplantation (FMT) is highly effective in preventing recurrent Clostridioides difficile infection (rCDI), yet its immunological mechanisms remain poorly defined. While bacterial engraftment and recovery of microbial diversity are central to FMT efficacy, accumulating evidence suggests that host immune reprogramming is involved. In murine models, regulatory CD4⁺ T cells are indispensable for clearing C. difficile. To address this mechanistic gap, we examined systemic immune reprogramming following FMT by performing flow cytometry and single-cell RNA sequencing (scRNA-seq) on a subset of successfully treated participants from a clinical trial comparing lyophilized FMT (LFMT) with lyophilized sterile fecal filtrate (LSFF, no live bacteria) for preventing rCDI. Flow cytometry was performed on peripheral mononuclear cells from 19 LFMT recipients and 18 LSFF recipients, and scRNA-seq analysis was performed on two LFMT recipients. Although flow cytometry results did not show significant changes in the assessed markers after rCDI resolution in either treatment group, exploratory scRNA-seq in the two LFMT recipients revealed distinct LFMT-associated transcriptional signatures across adaptive and innate-like lymphocyte populations. LFMT was associated with upregulated activation and regulatory genes (CD69, STAT1, TOX, RORA, FOXP3) in CD4⁺ and CD8⁺ T cells, suggesting enhanced immune regulation with reduced cytotoxic gene expression (GZMB, PRF1, GNLY). Innate-like lymphocytes displayed broad activation, with natural killer cells showing increased KLRD1, PRF1, and IL2RB and mucosal-associated invariant T cells (MAIT cells) upregulating STAT1, JUN, and RORA while downregulating KLRB1 and STAT3. These transcriptional programs are consistent with recalibration of T cell homeostasis and innate-like lymphocyte activation, potentially driven by microbial restoration. Collectively, this exploratory study provides the first single-cell immune atlas of LFMT in rCDI, identifying coordinated activation of regulatory, effector, and innate immune pathways. Given the small sample size, these findings should be considered hypothesis-generating, requiring validation in larger cohorts.