## 4 Discussion
The current literature examining IL-15 and OA has excluded patients with a history of traumatic joint injury ([PERSON] et al., 2017; [PERSON] et al., 2017; [PERSON] et al., 2017; [PERSON] et al., 2017), thus excluding PTOA, which could be another reason for the lack of significance between groups in our study. IL-15 may play a more vital role in other OA phenotypes, such as primary OA, where risk factors such as aging and metabolic syndrome may have effects on OA pathophysiology that involve IL-15, but were not examined in this study. Research utilizing _II15-/-_ mice on a high fat diet revealed an important role for IL-15 in metabolic syndrome, as it seems to promote chronic inflammation in adipose tissue ([PERSON] et al., 2017). Perhaps the inflammatory role of IL-15 is more substantial in metabolic OA than in PTOA. Finally, the redundant and pleiotropicnature of cytokines could explain why the absence of IL-15 did not cause a change in PTOA progression. This is partly due to the ability of cytokines to signal through multiple receptor complexes and shared receptors; for example, IL-15 has a shared \(\beta\) receptor with IL-2 and a common \(\gamma\) receptor shared by five interleukins [15]. IL-15 additionally activates similar signaling cascades to other cytokines, namely IL-2 [16]. Therefore, redundancy with other cytokines presented a challenge to our study. Additionally, the activity of IL-15 on catabolic factors, like MMPs, may be more indirect, as suggested by recent work by [PERSON] and colleagues (2020). Chondrocytes treated with IL-15 in vitro demonstrate a delayed release of MMP-1 and -3, compared to TNFa, which may point to an indirect effect of IL-15 on these MMPs [23]. Therefore, the action of IL-15 to increase catabolic activity in the joint may be taken over by another cytokine when IL-15 is absent, especially if this activity is indirect. IL-15 is critical for NK cell maturation and functional activity [16]. _II15\({}^{-/-}\)_ rats are therefore deficient in NK cells, which was successfully demonstrated by work from [PERSON] and colleagues (2017) [26]. Interestingly, NK cells have been demonstrated to be a main infiltrating lymphocyte in the synovial tissue of OA patients with a functionally distinct phenotype from peripheral blood NK cells. Further, OA synovial fluid contains both NK-cell chemoattractants and NK-cell activating cytokines [37]. Recent work has characterized the CD56+/CD16- NK cells as the main NK phenotype found in OA synovial fluid, expressing higher levels of granzyme A and correlating to higher pro-inflammatory cytokine levels [38]. Our work did not demonstrate a significant difference in the sub-synovial infiltration of _II15\({}^{-/-}\)_ rats even though there was a NK-cell deficiency. This may be due to previously discussed factors, such as species difference and timepoint, as the above mentioned studies examined human synovial tissue at the disease end stage. Regardless, a further examination of NK-cell activity in our rat PTOA model would be of great benefit. It is important to consider the limitations involved in this work. Many of these limitations were due to the laboratory shutdown during the COVID-19 pandemic, which intervened with completion of additional in vivo experiments. Our work also lacked immunohistochemistry (IHC) or other molecular data, we attempted to demonstrate IL-15 presence in the joint through IHC. Since IL-15 is mandatory for NK cell maturation we targeted perform, a pore-forming protein found in NK cells, as a surrogate marker for IL-15 activity [15; 16]. Unfortunately, we were not able to obtain convincing data using this approach. Future work would benefit from an optimized method of IL-15 detection in the joint through immunohistochemistry in order to assess the expression of IL-15 in the rodent joint. As previously discussed, exploring multiple timepoints and perhaps a metabolic or aging OA model could be beneficial to exploring the role of IL-15 in OA.