Inhibition of platelet deposition on leukocytes by administration of platelet-depleting antibodies or heparin reduces the anti-inflammatory effect of IM7 treatment in PGIA. (A) Deposition of platelets, seen as bright particles on leukocyte surfaces, in the synovial venule of a mouse pretreated with PBS (left) before IM7 injection, is inhibited in animals that received platelet-depleting pAb (middle) or heparin (right) 25 minutes before IM7 administration. (B) In control rat IgG-injected arthritic mice, platelet-depleting antibodies (▩) and heparin (■) do not have an effect on the 24-hour progression of joint swelling compared with PBS (□), but both platelet depletion and heparin significantly reduce the therapeutic effect (ie, suppression of joint swelling) of IM7. Mice were injected with PBS or received platelet-depleting pAb (100 μg) or heparin (40 U) 25 minutes before administration of rat IgG or IM7 (200 μg each). Joint thickness was measured at 0 and 24 hours. Results are expressed as changes (in mm) in cumulative (sum of 4 paws) joint thickness in 24 hours (n = 6 mice/group; *P < .05).