Peri-Operative Antimicrobial Prophylaxis Modulates CD4+ Lymphocyte Immunophenotype Ex Vivo in High-Risk Patients Undergoing Major Elective Surgery—A Preliminary Observational Study

<b>Background</b>: Post-operative infections are a significant cause of morbidity in patients undergoing major elective surgery. Peri-operative antibiotics are used to reduce the risk of infection. Several antibiotics modulate the host immune response. <b>Objectives</b>: Our objective was to determine the <i>ex vivo</i> immunomodulatory properties of commonly used antibiotics (amoxicillin, cefuroxime, metronidazole, or combined cefuroxime-metronidazole) on monocyte and lymphocyte phenotypes in patients undergoing major elective surgery. <b>Methods</b>: We performed a prospective cohort study of patients aged ≥18 years admitted to the post-anaesthetic care unit following major elective non-cardiac surgery. Peripheral blood mononuclear cells isolated immediately after surgery were incubated with antibiotics with or without a monocyte (heat-killed <i>E. coli</i>) or lymphocyte (CD3/CD28 beads) stimulus <i>ex vivo</i>. Immune cell phenotype was characterised using flow cytometry. <b>Results</b>: Twenty-eight patients were included. All antibiotics tested were associated with a reduction in T-cell viability, and changes to monocytes were minimal. Among CD4⁺ and CD8⁺ lymphocytes, cefuroxime increased IFN-γ (at low and high doses) and increased CD4⁺ lymphocyte IL-2 and IL-2R at higher doses. Among CD4⁺ lymphocytes, at both doses, cefuroxime increased %T_h1 population, with a parallel decrease in %T_h2, %T_h17, IL-17A, FOX-P3, and T-bet. Among the T_h1 sub-population, changes were seen at higher cefuroxime doses, including increased viability and PD-1, and a decrease in FAS, IFN-γ and CD28, and IL-7R expression. <b>Conclusions</b>: The choice of antibiotics directly impacts immune function following major surgery, with cefuroxime associated with <i>ex vivo</i> immunomodulatory effects on CD4⁺ lymphocytes. The functional implications on the development of subsequent post-operative infectious complications and long-term cancer-free survival require further investigation.