Background/Purpose: Macrophages in rheumatoid arthritis (RA) synovium produce high levels of inflammatory cytokines/chemokines and play the pivotal role in promoting inflammation and joint destruction. Treatment of patients with RA with infliximab results in a rapid reduction of synovial tissue macrophages, which is a reliable biomarker of clinical response. Since the mechanism for reduction of macrophages has not been clearly defined, studies were performed with human TNF transgenic (hTNF-tg) mice to determine if the initial reduction of macrophages was due to increased cell death, increased efflux of macrophages or decreased influx of monocytes.

Methods: hTNF-tg mice and littermate controls at 5-6 week old were employed. Administration of infliximab was performed intraperitoneally (10mg/kg), 2 doses over 3 days. The clinical severity of the arthritis was defined as the sum score of inflammation, deformity and grip strength, ranging 0-28. Inflammation and bone destruction were assessed by ankle histology. The immune cell phenotypes and apoptosis were determined by flow cytometry. Monocyte migration into ankles was documented following intravenous administration of CD115⁺ monocytes from CD45.1⁺donors into the synovial tissue of hTNF-tg mice with or without the infliximab treatment. Ankle joint homogenizes were analyzed for cytokines/chemokines by quantitative ELISA.

Results: Arthritis in 5 week old CD45.2 hTNF-tg mice was significantly improved after 2 dosages of infliximab evaluated clinically and histologically, which demonstrated reduction of inflammation and bone erosion. In addition, the Ly6C⁺ synovial tissue monocyte-derived macrophages (CD64⁺CD11b⁺ F4/80^mid Ly6C⁺) were sighnificantly reduced by infliximab (p<0.001), determined by flow cytometric analysis. No increase of macrophage apoptosis was identified following treatment, although there was a modest reduction neutrophil apoptosis. There was no increase of any subset of macrophages in the draining peritoneal lymph nodes (pLNs) of the treated mice. In fact the Ly6C⁺ macrophages were actually reduced in the pLNs following treatment. The adoptive transfer CD45.1 monocytes was employed to track the influx of monocytes. The influx of CD115⁺circulating monocytes into the synovial tissue of hTNF-tg mice was significantly (p < 0.001) reduced in the mice that received 2 doses of infliximab, one 24h before and another at the time of monocyte transfer, or just one dose simultaneously with monocyte administration (p<0.001). Ankle joint homogenizes demonstrated greatly reduced the CCL2 (p<0.001), but not Cx3CL1 (p>0.05), following treatment.

Conclusion: The initial effect of infliximab on macrophages in the joints of hTNF-tg mice was the reduction of Ly6C+ macrophages. The decrease was not due to increased efflux from the joint or increased apoptosis but due to decreased influx of circulating Ly6C+ monocytes, which are also CCR2+. These observations provide a potential explanation for the rapid reduction of synovial tissue macrophages observed in patients with RA treated with infliximab.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/infliximab-suppresses-the-monocyte-chemotaxis-in-human-tnf-transgenic-mice/