Soluble CD26: From Suggested Biomarker for Cancer Diagnosis to Plausible Marker for Dynamic Monitoring of Immunotherapy.

Simple Summary: Prognostic markers have become a promising tool for predicting response to a certain treatment and qualifying patients for a certain therapy; however, it is important to consider that they generally describe a probabilistic scenario. For instance, the analysis of programmed death ligand 1 expression on tumor cells or tumor mutational burden is commonly employed as a biomarker with relative predictive value for immunotherapy efficacy in non-small-cell lung cancer (NSCLC) patients. Cutting-edge techniques, such as next-generation sequencing of tumor tissue and flow cytometry analysis of lymphocyte subpopulations in the peripheral blood of patients, are used with the aim of increasing the predictive value, avoiding statistical weakness and discovering algorithms based on many biomarkers. To resolve the issues posed by these novel techniques, dynamic monitoring of biomarker changes can indicate resistance or response to immunotherapy during different points of the treatment. Hence, clinicians can flexibly modify the therapy course for the benefit of the patient, improving response rates, efficiency in laboratory routines, and the economic cost of the technique. Immune checkpoint immunotherapy alters the local (solid tumor infiltration) and systemic balance among several immune system cell populations. The usefulness of immune-cell-related soluble CD26 and its DPP4 activity in immunotherapy monitoring is discussed. Soluble CD26 (sCD26), a glycoprotein with dipeptidyl peptidase (DPP4) enzymatic activity, can contribute to early diagnosis of colorectal cancer and advanced adenomas and has been studied, including for prognostic purposes, across various other types of cancer and disease. The latest research in this field has confirmed that most, though not all, serum/plasma sCD26 is related to inflammation. The shedding and/or secretion of sCD26 from different immune cells are being investigated, and blood DPP4 activity levels do not correlate very strongly with protein titers. Some of the main substrates of this enzyme are key chemokines involved in immune cell migration, and both soluble and cell-surface CD26 can bind adenosine deaminase (ADA), an enzyme involved in the metabolism of immunosuppressor extracellular adenosine. Of note, there are T cells enriched in CD26 expression and, in mice tumor models, tumor infiltrating lymphocytes exhibited heightened percentages of CD26+ correlating with tumor regression. We employed sCD26 as a biomarker in the follow-up after curative resection of colorectal cancer for the early detection of tumor recurrence. Changes after treatment with different biological disease-modifying antirheumatic drugs, including Ig-CTLA4, were also observed in rheumatoid arthritis. Serum soluble CD26/DPP4 titer variation has recently been proposed as a potential prognostic biomarker after a phase I trial in cancer immunotherapy with a humanized anti-CD26 antibody. We propose that dynamic monitoring of sCD26/DPP4 changes, in addition to well-known inflammatory biomarkers such as CRP already in use as informative for immune checkpoint immunotherapy, may indicate resistance or response during the successive steps of the treatment. As tumor cells expressing CD26 can also produce sCD26, the possibility of sorting immune- from non-immune-system-originated sCD26 is discussed. [ABSTRACT FROM AUTHOR]