Differential chemotactic potential of mouse platelet basic protein for thymocyte subsets

Mouse platelet basic protein (CXCL7/mPBP) was cloned from thymic stromal cells and further identification indicated that it was expressed in thymic monocytes/macrophages (Mo/Mφs). Recombinant mPBP was chemoattractive for target cells of polymorphonuclear leucocytes, peritoneal Mo/Mφs and splenic lymphocytes with distinct potencies. CXCR2 was identified to be a cognate receptor for mPBP. Mouse thymocyte subsets of CD4-CD8- double-negative (DN), CD4⁺CD8⁺ double-positive (DP), CD4+CD8 single-positive (CD4SP) and CD4-CD8⁺ single-positive (CD8SP) expressed cell surface CXCR2 with different positive percentages and expression levels. mPBP was chemoattractive for thymocyte subsets with the potency order DN>DP> CD8SP>CD4SP, consistent with the levels of CXCR2 expressed on the respective cells. Thus, mPBP in thymus is functionally redundant with chemokine CXCL12/ SDF-1. Moreover, our finding that thymic Mo/Mφs can produce mPBP implies that they may have other functions apart from acting as scavengers in thymus.