Objective: To evaluate interactions of human intravenous immunoglobulin (IVIG) with canine lymphocytes and monocytes.
Sample population: Heparinized blood samples from 4 clinically normal Beagles.
Procedure: Binding ability of IVIG to canine lymphocytes and monocytes was measured by flow cytometry and an indirect immunofluorescent assay. Dual-staining fluorescent assays were done to determine lymphocyte subsets that bind IVIG. Competitive assays were done, using intact canine IgG and Fc fragments, and inhibition of binding was compared with that of F(ab)2 fragments. Ability of IVIG to inhibit phagocytosis of antibody-coated canine RBC also was determined, using a canine mononuclear cell phagocytic assay.
Results: IVIG concentrations (10, 1, 0.1, and 0.01 mg/ml) bound to (mean+/-SD) 99.6+/-0.4, 92.4+/-6.1, 20.4+/-24.6 and 2.0+/-5.1 % of canine lymphocytes, respectively, Dual staining analyses with IVIG and canine lymphocyte markers indicated that IVIG bound to CD4, CD8, and B lymphocytes. The aforementioned 4 IVIG concentrations bound to 98.0+/-2.1, 85.5+/-13.5, 64.7+/-32.8, and 26.5+/-17.1 % of monocytes, respectively. Inhibition of IVIG (0.01 mg/ml) binding to monocytes was significant (P< 0.05) in the presence of 1 and 10 mg of canine IgG/ml and 1 mg of canine Fc fragments/ml. In the presence of F(ab')2 fragments of canine IgG, inhibition was not significant, suggesting that binding is Fc mediated. Co-culturing of monocytes, opsonized RBC, and the 4 concentrations of IVIG and no IVIG resulted in 11.8+/-5.1, 27.7+/-12.3, 31.8+15.1, 53.8+/-6.7, and 45 + 12% of the monocytes containing RBC, respectively. Phagocytosis inhibition was significant (P < 0.05) at an IVIG concentration of 10 mg/ml.
Conclusions: IVIG binds to canine lymphocytes and monocytes; binding to the latter is Fc mediated. IVIG also inhibits Fc-mediated phagocytosis of antibody-coated RBC.
Clinical relevance: Owing to its ability to inhibit Fc-mediated phagocytosis of antibody-coated RBC, IVIG may be an effective short-term treatment for dogs with immune-mediated hemolytic anemia.