Flow cytometric identification of dynamic immune cell populations in a rat model of post-traumatic elbow contracture

Purpose/Aim: Post-traumatic joint contracture (PTJC) commonly occurs after elbow injury. Previous findings implied immune system activation in capsules of contracted joints; however, quantifying immune cell populations in rodent joint tissues is challenging due to small size and low cellularity. Here, we used flow cytometry to investigate the temporal immune response and enumerate cell populations in the rat elbow capsule after traumatic injury. Materials and Methods: After inducing PTJC, capsules were harvested from injured, sham, and control rat elbows at multiple time points, stained with surface markers for immune cells, and quantified via flow cytometry. Results were compared to previously published mechanics and histology data from the same model. Another injured group was treated with celecoxib to determine if changes due to anti-inflammatory treatment could be detected. Results: Compared to control, injured animals displayed elevated leukocytes, T cells, and natural killer cells. CD45+ cells exhibited similar temporal changes as mechanics, which increased and then decreased after injury. CD3+, CD4+, and CD8a+ T cells followed a similar pattern as histology scores, which increased and remained elevated. Treating injured animals with celecoxib increased leukocytes but decreased several immune subpopulations. Conclusions: A method for flow cytometry on rat elbow capsule was established and used to quantify immune cell populations, which changed in response to injury and anti-inflammatory treatment. Comparisons between flow cytometry and previously published mechanics and histology revealed additional insights about temporal patterns in cell-, tissue-, and joint-level changes. Future work will investigate whether changes in immune cells attenuate PTJC symptoms.