Abstract Background and Aims IgM antibodies are not typically detected using traditional crossmatch techniques, and the literature on pre-transplant anti-HLA IgM antibodies remains limited. Consequently, their role in post-transplant outcomes is not yet fully understood. However, existing studies suggest a lower incidence of acute graft rejection in patients with IgM-positive crossmatches. This effect may be partially explained by studies proposing a potential immunoregulatory role of IgM immunoglobulins. The objective of our study was to evaluate post-transplant outcomes based on the presence of anti-HLA IgM antibodies detected during pre-transplant screening. Method This retrospective, single-center study analyzed living-donor kidney transplants performed at Gemelli Hospital in Rome, Italy, over a 5-year period (January 2019–December 2023). The objective was to evaluate post-transplant outcomes based on the presence of anti-HLA IgM antibodies detected during pre-transplant screening. Patients were divided into three groups: (1) anti-HLA IgM-negative, (2) non-donor-specific (DSA) anti-HLA IgM antibodies, and (3) DSA anti-HLA IgM-positive. The primary endpoint was one year graft function, assessed by the eGFR slope and the presence of proteinuria. Secondary endpoints included incidence of rejection, delayed graft function (DGF), infections and tumor during follow-up (until December 2024). Statistical analyses included Shapiro-Wilk, ANOVA, Kruskal-Wallis, and Chi-square tests. Results A total of 116 transplants were performed, 80 patients anti-HLA IgM-negative, 14 with non-DSA anti-HLA IgM, and 22 with DSA anti-HLA IgM. There were no differences in the use of induction and maintenance immunosuppression in the 3 study groups. There were six cases of DGF in group 1 and none in group 2 and 3 (P = 0.04). Group 3 patients had higher eGFR values throughout 1 year follow-up compared to group 1 and 2, although the differences were not statistically significant. Group 2 patients were characterized by a higher incidence of anti-HLA IgG DSA after transplantation (P = 0.0002). Notably, in 54.5% of group 3 patients IgM anti-HLA DSA disappeared during follow-up, whereas only 4.5% developed IgG anti-HLA DSA corresponding to pre-transplant IgM. We observed 4 acute antibody-mediated or cell-mediated acute rejection in group 1, 2 mixed rejections in group 2 and no rejections in group 3 (P = 0.01). Incidence of nephropathy recurrence was similar in the three groups. Finally, group 3 patients exhibited a higher incidence of CMV (P = 0.04) and BK virus (P = 0.023) infections. However, the incidence of other infections and post-transplant neoplasia was similar across groups. Conclusion Our findings suggest a protective role of donor-specific IgM antibodies in reducing acute rejection incidence and other transplant complications, ultimately supporting improved graft survival during follow-up.
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