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

Abstract

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.

Keywords: ADA (adenosine deaminase); CD26; DPP4 (dipeptidyl peptidase 4); T cells; biomarker; chemokines; immunotherapy; monitoring; sCD26 (soluble CD26); tumor cells.

Figure 1

(A) Sensitivity of sCD26 (black boxes) and CEA serum levels for the diagnosis of colorectal cancer in the four Dukes’ stages (12, 55, 29, 14 samples, respectively) [45]. (B) Normalization of the sCD26 levels before (black bars) and after polypectomy (white bars) in four cases diagnosed with polyps. A 410 ng/mL cut-off point is denoted by a discontinuous line [46].

Figure 2

Follow-up sCD26 levels [117]. (a,b) Two representative disease-free patients (c,d) with tumor recurrence, (e) one representative patient with tumor persistence, and (f) one who developed hepatic and (g,h) pulmonary metastasis. Black arrows indicate the beginning and end of chemotherapy cycles; the upward arrow indicates diagnosis of metastasis, and the dashed arrow, exitus.

Figure 3

Schematic representation of the behavior of sCD26 during follow-up of CRC patients according to the disease status [117].

Figure 4

Dipeptidyl peptidase 4 (DPP-IV) enzymatic activity levels (mean and SD, U L⁻¹) (A) and sCD26 concentration levels (mean and SD, g L⁻¹) (B) in serum of RA patients grouped according to the type of therapy and healthy donors [88]. N = number of samples; (1) HC, healthy donors n = 25; (2) cDMARD, conventional disease modifying anti-rheumatic disease, n = 21; bDMARDs (3) anti-TNF, n = 58; (4) anti-CD20, n = 12; (5) anti-IL6R, n = 11; (6) IgCTLA4, n = 4.

Figure 5

Western blot analysis of sCD26 presence in the serum of two donors. The membranes were developed with polyclonal rabbit anti-CD26 Ab ((left), Novus Biologicals), polyclonal goat anti-CD26 Ab ((centre), RnD Systems), and mAb antiCD26 ((right), Immunostep, Salamanca, Spain). Additionally, 20 μg of protein in each line was treated at 37 °C for 15 min before SDS-PAGE electrophoresis, (there were no recognized Ab bands from samples treated at 100 °C). Data shown are representative of at least three experiments. For comparison, the molecular weight of the main detected bands in each gel is shown below them and the black line shows the expected weight of sCD26 (Supplemental Materials shows the original gels: Figure S1).