OBJECTIVE: To determine polarization of synovial macrophages during development of cruciate ligament rupture (CR) and determine whether differences in synovial macrophage polarization in CR, osteoarthritis (OA), and healthy joints exist.
STUDY DESIGN: Prospective case-controlled study.
ANIMALS: Client-owned dogs with unstable stifles with CR (n = 22), paired stable contralateral stifles with partial CR (pCR; n = 7), joints with OA not related to CR (n = 6), and clinically normal (Normal; n = 7) joints.
METHODS: Synovial fluid samples were collected. Smears were made for differential cytology counts and estimated total nucleated cell counts. Cytospin preparations were made, and immunocytochemical staining was performed with the pan-macrophage marker CD68, M1 macrophage markers inducible nitric oxide synthase (iNOS) and chemokine (C-C motif) receptor 7 (CCR7), and M2 macrophage markers arginase 1 and CD163. Positively stained cells were counted.
RESULTS: Numbers of lymphocytes were increased in the CR group compared with the OA and Normal groups (P < .05). Numbers of CD68+ , CCR7+ , and iNOS+ cells in the CR and OA groups were increased compared with the Normal group (P < .05). Globally, the ratio of positively stained M1 polarized CD68+ cells to M2 polarized CD68+ cells was highest for the OA group (2.49), followed by the pCR (2.1), CR (1.63), and Normal (0.7) groups.
CONCLUSION: Polarization of synovial macrophages toward an M1 proinflammatory phenotype is an early event in the development of canine CR.
CLINICAL SIGNIFICANCE: M1 polarization in pCR stifles provides evidence of a possible role for macrophages in progressive development of cruciate ligament fiber damage. Lymphocytes may play a role in the synovitis found in CR joints. Our findings provide evidence that these cells are therapeutic targets.