A novel canine leukointegrin, α_dβ₂, is expressed by specific macrophage subpopulations in tissue and a minor CD8⁺ lymphocyte subpopulation in peripheral blood

The β₂ or leukointegrin family is comprised of three structurally related leukocyte surface heterodimers: LFA-1 (CD11a/CD18), Mac-1/Mo-1 (CD11b/CD18), and p150,95 (CD11c/CD18). In this work, we describe a novel canine β₂ (CD18)-associated leukointegrin, designated α_d. Expression of α_d in tissues was prominent in macrophages in splenic red pulp, lymph node medullary regions, and bone marrow. In peripheral blood, α_d expression was limited to a minor subpopulation of CD8⁺ T cells, which included small lymphocytes and large granular lymphocytes. A minor subpopulation of either CD8⁺ or CD4^-CD8^- splenic red pulp lymphocytes also expressed α_d. Immunoprecipitation of α_d from canine splenocytes revealed a heterodimer of 155 kDa and 95 kDa. Prior clearance of splenocyte extracts with an anti-CD18 mAb resulted in complete removal of α_d. In addition, prior clearance of canine splenocyte extracts with anti-CD11a, anti-CD11b, or anti-CD11c mAb failed to clear α_d. These immunoclearance data indicated that canine α_d was antigenically distinct from the three known CD11 molecules, and occurred as an α_dβ₂ heterodimer. Amino acid sequencing of canine α_d affinity isolated from spleen further suggested that canine α_dβ₂ probably represented a fourth member of the canine leukointegrin family via its homology to a subsequently discovered, novel human leukointegrin, α_dβ₂, which further supported the uniqueness of the canine protein. The discovery of canine α_d, and the demonstration of its highly restricted cell and tissue distribution, support a re-evaluation of leukointegrin-dependent inflammatory and immunologic interactions that involve cells now known to express α_d.