## 3 Results
### Synovitis Is Not Affected by IL15 Genotype  
Synovitis was similarly assessed using a semi-quantitative six-parameter scoring system. A score from 0 (none) to 3 (severe) was given across six compartments (medial and lateral parapatellar, superior, and inferior compartments) for six parameters: (1) synovial lining thickness, (2) sub-synovial infiltration, (3) surface fibrin deposition, (4) vascularization, (5) fibrosis, and (6) perivascular edema. Inter-rater reliability testing reveals substantial to almost perfect agreement, with a weighted kappa ranging from 0.67 to 0.85 across the six parameters [31]. As expected, signs of severe synovitis including vascularization, fibrosis, and vascular edema were similar between PTOA and Naive groups at this early stage of PTOA development. In keeping with early stage PTOA development,  
Figure 5: Cartilage damage is not significantly different between _II15\({}^{+/+}\)_ PTOA and _III5\({}^{-/-}\)_ PTOA rats. Animals either underwent ACLT-DMM surgery to induce PTOA or served as control with no surgery (Naive). Semi-quantitative scoring (scale 0–5) of toluidine blue-stained paraffin sections was used to determine the extent of cartilage damage in the joint via 4 compartments; medial femoral condyle (MFC), medial tibial plateau (MTP), lateral femoral condyle (LFC), and lateral tibial plateau (LTP). Two-way ANOVA with [PERSON]’s multiple comparison test was used for statistical analysis; data represents mean with 95% CL There was no significant difference in the summed scores between the _III5\({}^{+/+}\)_ PTOA and _III5\({}^{-/-}\)_ PTOA groups. The PTOA groups scored significantly higher than the Naive groups in the medial compartment. (***\(p<0.001\), ***\(p<0.0001\), N = 15 rats/PTOA group and 9 rats/Naive group). only the sub-synovial infiltrate and surface fibrin scores were increased in PTOA animals. However, there was no clear difference between genetic strains (Figure 8). There was no significant statistical difference between the _Ill5\({}^{+/+}\)_ and _Ill15\({}^{-/-}\)_ PTOA rats across all six parameters (Figure 9). Overall, synovitis did not clearly differ between the _Ill5\({}^{+/+}\)_ PTOA and _Ill15\({}^{-/-}\)_ PTOA rats 8 weeks after PTOA induction. Figure 6: Subchondral bone damage is not significantly different between _Ill15\({}^{+/+}\)_ PTOA and _Ill15\({}^{-/-}\)_ PTOA rats. Rats either underwent ACLT-DMM surgery to induce PTOA or served as a control with no surgery (Naive). Semi-quantitative scoring (scale 0-5) was used to determine the extent of subchondral bone damage in the joint via 4 compartments: medial femoral condyle (MFC), medial tibial plateau (MTP), lateral femoral condyle (LFC), and lateral tibial plateau (LTP). Two-way ANOVA with [PERSON]’s multiple comparison test was used for statistical analysis; data represents mean with 95% CI. There was no significant difference in summed scores between the _Ill5\({}^{+/+}\)_ PTOA and _Ill15\({}^{-/-}\)_ PTOA groups. The PTOA groups scored significantly higher than the Naive groups only in the MTP. (\({}^{***}\)_p_\(<\) 0.0001, N = 15 rats/PTOA group and 9 rats/Naive). Figure 8: Representative images of synovitis in the PTOA group. Histological images of the synovitis, as demonstrated by surface fibrin and sub-synovial infiltration, found in the PTOA animals. Healthy synovium (first box) is presented as a comparison and represents Naive tissue. Surface fibrin (red arrow) and sub-synovial infiltration (red arrowheads) were found significantly more in the PTOA group compared to the Naive, but was not different between genetic variants. Images take at 20\(\times\) magnification, scale bars represent 100 \(\upmu\)m (N = 15 rats/PTOA group and N = 9 rats/Naive group). Figure 9: Synovitis is not significantly different between _Ill15\({}^{+/*}\)_ PTOA and _Ill15\({}^{-/-}\)_ PTOA rats. Rats either underwent ACLT-DMM surgery to induce PTOA or served as control with no surgery (Naive).