Identification and Biochemical Characterization of Human Plasma Soluble IL-7R: Lower Concentrations in HIV-1-Infected Patients

Abstract

The IL-7R α-chain and the common γ-chain (γc) are both components of IL-7R. Human plasma harbors soluble forms of IL-7R (sIL-7Rα and sγc) that are detected and assayed by Western blotting, showing that the levels of sIL-7Rα are higher than the levels of sγc (47.5 ng/ml and 1.5 ng/ml, respectively). Gel electrophoresis and tandem mass spectrometry used to analyze deglycosylated, affinity-purified protein showed that sIL-7Rα is generated through differentially spliced mRNA, not by membrane receptor shedding. Plasma sIL-7Rα and sγc are present as heterocomplexes and sγc was found to be mainly associated with sIL-7Rα. The affinities of two IL-7 binding sites (Kd = 35 ± 8 pM and Kd = 3 ± 1 nM) were similar to that of the membrane receptor, suggesting that the sIL-7Rα/sγc complex retains high affinity for IL-7. sIL-7Rα mRNA is constitutively present among peripheral T lymphocytes and is down-modulated in vitro by IL-7. Chronically HIV-1-infected patients (n = 20) showed no significant (p > 0.714) variation in sγc levels and a significant (p < 0.0014) 2-fold decrease in plasma sIL-7Rα levels compared with those in control healthy individuals. Plasma IL-7 and sIL-7Rα levels did not show any obvious relationship.