Chemokines constitute a large family of chemotactic cytokines that selectively attract different blood cell types. Although most inflammatory chemoattractants are only induced and released in the circulation during acute infection, a restricted number of CXC and CC chemokines are constitutively present in normal plasma at high concentrations. Here, such a chemotactic protein was purified to homogeneity from serum and fully identified as a novel CC chemokine by mass spectrometry and amino acid sequence analysis. The protein, tentatively designated Regakine-1, shows less than 50% sequence identity with any known chemokine. This novel CC chemokine chemoattracts both neutrophils and lymphocytes but not monocytes or eosinophils. Its modest chemotactic potency but high blood concentration is similar to that of other chemokines present in the circulation, such as hemofiltrate CC chemokine-1, platelet factor-4, and â€-thromboglobulin. Regakine-1 did not induce neutrophil chemokinesis. However, it synergized with the CXC chemokines interleukin-8 and granulocyte chemotactic protein-2, and the CC chemokine monocyte chemotactic protein-3, resulting in an at least a 2-fold increase of the neutrophil and lymphocyte chemotactic response, respectively. The biologic effects of homogeneous natural Regakine-1 were confirmed with chemically synthesized chemokine. Like other plasma chemokines, it is expected that Regakine-1 plays a unique role in the circulation during normal or pathologic conditions.
IntroductionSerum is a rich source of leukocyte chemotactic factors that influence the migration of different leukocytic cell types to and from the blood circulation. For example, anaphylatoxin, or C5a, a cleavage product formed during complement activation, chemoattracts both polymorphonuclear and mononuclear blood cells. Other serum proteins such as platelet factor-4 and neutrophil-activating protein-2 are thrombocytederived chemotactic cytokines belonging to the chemokine family. [1][2][3][4] In contrast to C5a, these and other chemokines are each selectively attracting a defined set of leukocytic cell types. The chemokine family is subdivided into 2 major classes, ie, CXC and CC chemokines depending on the positioning of conserved cysteine residues. 1-5 The spectrum of target cells for each chemokine depends on the expression of one or more specific receptors on the different leukocyte subtypes. The receptors of all chemokines as well as those of C5a and other chemoattractants such as leukotriene B 4 and bacterial N-formylmethionyl-containing peptides belong to the family of G protein-coupled 7-transmembrane domain receptors. 6 Addition of bovine or human serum is often essential for the growth or maintenance of continuous and primary cell cultures. For example, we and others have used in the past low serum concentrations to preserve high viability of freshly isolated human leukocytes or to support the growth of hematopoietic progenitor cells in well-defined media. 7 Conflicting or variable experimental results have often been rela...