Soluble chemokine CCR5 receptor is present in human plasma

In view of the natural resistance to infection by HIV and occasional delayed clinical manifestation of the disease, as also the fact that the virus is able to enter only cells that express CD4 and a co-receptor, we initiated a search for a soluble co-receptor that might compete with its membrane counterpart. Using a sandwich ELISA system, a soluble human CCR5 receptor (sCCR5) was indeed detected in the circulation. Immunoprecipitation of sCCR5-positive plasma samples from Israelis of Ethiopian and non-Ethiopian origin with mAb 2D7, a conformation-dependent anti-CCR5 antibody, revealed the presence of a ∼22 kDa protein. A panel of antibodies directed against the membrane receptor was used to characterize the structure of the soluble CCR5: mAb CTC8, recognizing the N-terminal sequence of the protein, ¹⁰YDIN ¹³; "multidomain" mAbs FAB181B and FAB183B that are dependent upon the presence of Q ⁹³ and D ⁹⁵ in ECL1 and K ¹⁷¹ and E ¹⁷² in ECL2A, and mAb FAB182B, recognizing the stretch ¹⁸⁴YSQYQF ¹⁸⁹, which spans the C-terminal part of the second extracellular loop. The presence of short soluble CCR5 in human plasma has not been previously described. Among HIV-negative non-Ethiopian Israelis, 20.4% were sCCR5-positive, as against only 10.5% in HIV-positives. However, 7.1% of HIV-negative Ethiopian Israelis were sCCR5 positive, as were 5.6% HIV-positives. Plasma concentrations of MIP-1β, the CCR5 agonist, were twice as high in sCCR5-positives (140.8 ± 25.8 pg/ml) as in the sCCR5-negatives (77.6 ± 11.0 pg/ml, P = 0.0157). A significant positive correlation between plasma levels of sCCR5 and MIP-1β was found (Fig. 4, r = 0.8, P < 0.0001).