Neopterin as a Biomarker of Cellular Immune Response in Renal Allograft Rejection Subtypes: Linking Cytokines and Immune Cells to Improve Diagnostic and Therapeutic Approaches

Background: Renal allograft rejection remains a major challenge in transplantation. Current diagnostic approaches, including biopsies, are invasive and may fail to detect subclinical immune activation, potentially contributing to progressive graft dysfunction. Reliable, non-invasive biomarkers capable of monitoring immune activation and distinguishing rejection phenotypes are therefore needed. Methods: In this retrospective study, we evaluated serum neopterin as a biomarker of immune activation and graft status over 12 months following transplantation. Associations between neopterin levels and immune parameters, including natural killer (NK)-to-CD3+CD16/CD56+ T cell ratios, cytokines (IFN-γ and IL-10), and CD4+CD25+FoxP3+ T cell frequencies, were assessed. A total of 211 first renal allograft recipients were followed longitudinally, including patients with acute rejection (AR) and matched stable graft (SG) recipients. Serum neopterin was quantified by enzyme immunoassay, and immunophenotyping, mRNA expression, and cytokine profiling were performed on peripheral blood samples. Results: Serum neopterin levels were significantly elevated in AR compared to SG recipients, with a threshold of 57 nmol/L distinguishing AR with 81% sensitivity and 80% specificity. While IFN-γ demonstrated higher diagnostic performance in cross-sectional analysis, neopterin showed a more sustained elevation over time, remaining increased in AR recipients even at later post-transplant time points. Neopterin correlated positively with IFN-γ, but not IL-10, and inversely with CD4+CD25+FoxP3+ T cell frequency. NK cells were enriched during rejection, whereas CD3+CD16/CD56+ T cells were more prominent in graft stability. The NK-to-CD3+CD16/CD56+ T cell ratio was highest during acute cellular rejection. Conclusions: Neopterin reflects Th1-associated immune activation in renal allograft recipients and provides a temporally stable, non-invasive marker of immune status. Although it does not outperform IFN-γ levels at the time of rejection, its stability and sustained elevation suggest a complementary role in longitudinal monitoring. Integration of neopterin with immune parameters, including cytokine profiles and cellular subsets, may enhance the assessment of graft immunological status and support clinical decision-making.