We have used the delayed-type hypersensitivity (DTH) response to SRBC or tuberculin to examine the role of the murine type 3 complement receptor in T lymphocyte-dependent inflammatory recruitment. Intravenous injection of 5C6, a CR3-specific rat mAb known to impair myelomonocytic adhesion, divided the DTH to SRBC in actively immunized mice into two phases. The early phase, which lasted 24 h, was characterized by maximal oedema and maximal inflammatory recruitment and was 5C6 inhibitable. The later phase was 5C6 resistant and reached a peak 48 h after antigenic challenge and was superimposable on the declining peak seen in control mice. Passive transfer of reactive T cells mixed with antigen was used to examine the myelomonocytic effector arm of the DTH alone. Both passive transfer of cutaneous DTH to SRBC and passive transfer of the largely monocytic T cell-dependent recruitment to tuberculin in the peritoneal cavity were completely abolished by systemic 5C6 treatment. Injection of 5C6-treated donor leukocytes at the site of passive transfer had no effect. Treatment of donor mice with 5C6 at the time of active immunization did not alter their ability to provide reactive T cells for passive transfer. The myelomonocyte-restricted rat mAb 7/4 and the rapidly cleared F(ab')2 fragment of 5C6 showed no inhibition of the DTH. In all cases, inhibition of footpad swelling correlated with histological evidence of inhibition of myelomonocytic cell recruitment. Peritoneal cell counts after local DTH to tuberculin showed complete inhibition of monocyte recruitment. We conclude that CR3 plays a quantitatively important role in T cell-dependent inflammatory recruitment. This is absolute in passive transfer experiments, but only partial after active immunization. Leukocyte CR3 plays a common role in both immunologically specific and nonspecific inflammatory recruitment and provides a target that could possibly be manipulated to therapeutic advantage.

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