Monoclonal antibodies specific to human Δ42PD1: A novel immunoregulator potentially involved in HIV-1 and tumor pathogenesis

We recently reported the identification of Delta 42PD1, a novel alternatively spliced isoform of human PD1 that induces the production of pro-inflammatory cytokines from human peripheral blood mononuclear cells and enhances HIV-specific CD8(+) T cell immunity in mice when engineered in a fusion DNA vaccine. The detailed functional study of Delta 42PD1, however, has been hampered due to the lack of a specific monoclonal antibody (mAb). In this study, we generated 2 high-affinity mAbs, clones CH34 (IgG2b) and CH101 (IgG1), from Delta 42PD1-immunized mice. They recognize distinct domains of Delta 42PD1 as determined by a yeast surface-displaying assay and ELISA. Moreover, they recognize native Delta 42PD1 specifically, but not PD1, on cell surfaces by both flow cytometry and immunohistochemical assays. Delta 42PD1 appeared to be expressed constitutively on healthy human CD14(+) monocytes, but its level of expression was down-regulated significantly during chronic HIV-1 infection. Since the level of Delta 42PD1 expression on CD14(+) monocytes was negatively correlated with the CD4 count of untreated patients in a cross-sectional study, Delta 42PD1 may play a role in HIV-1 pathogenesis. Lastly, when examining Delta 42PD1 expression in human esophageal squamous-cell carcinoma tissues, we found high-level expression of Delta 42PD1 on a subset of tumor-infiltrating T cells. Our study, therefore, resulted in 2 Delta 42PD1-specific mAbs that can be used to further investigate Delta 42PD1, a novel immune regulatory protein implicated in HIV-1 and tumor pathogenesis as well as other immune diseases.