Anne Lippinkhof

Osteoarthritis (OA) is a debilitating joint disorder characterized by the loss of articular cartilage, leading to stiffness and pain. The incidence of OA is increasing due to aging and the rise in obesity. This thesis aims to investigate how metabolic overload contributes to the pathophysiology of knee OA, with a focus on the interplay between metabolism and immunity.

In Chapter 2, we evaluated the robustness and repeatability of the diet-induced OA model. We demonstrated that long-term feeding with a high-caloric diet consistently induces a metabolic phenotype (such as obesity and glucose intolerance), but the severity of cartilage damage varies by mouse strain. The most consistent cartilage damage was observed in mice with genetic modifications leading to a human pro-inflammatory status (human C-reactive protein (hCRP) mice) or altered lipoprotein metabolism (ApoE*3Leiden.CETP mice). We therefore suggest that an additional trigger, besides high-caloric food, is needed to induce metabolic OA.

To evaluate whether a high-fat diet indeed aggravates the development of OA, low-fat and high-fat feeding were combined with a mild mechanical stressor (Chapter 3). OA was induced by microsurgical destabilization of the medial meniscus (DMM) in wild-type C57BL/6J mice. This post-traumatic OA (PTOA) model is low-invasive and sensitive enough to study subtle changes in disease progression caused by mild triggers such as genetic background and diet. High-fat diet (HFD) indeed increased the severity of cartilage damage in this model, and our findings point to a role for both lipid dysregulation and monocyte activation in OA progression. Based on these results, we propose that these systemic factors contribute to diet-induced inflammation that enables the aggravation of cartilage damage.

To unravel the effects of dysregulated lipid metabolism on inflammatory status, we studied the development of metabolic inflammation both locally (in the knee joint) and systemically (in the blood). In Chapter 4, an extension of the studies in Chapters 3 and 5, we focused on the inflammatory status of the infrapatellar fat pad (IFP) during OA development. Evidence suggests the IFP may be a potential local source of inflammatory mediators in the knee and thus contribute to OA progression. The most pronounced fat inflammation, characterized by 'crown-like structures' (CLS), was found in mice receiving a high-fat diet combined with a surgical trigger (DMM). We observed significantly increased deposition of the macrophage marker iNOS due to DMM surgery, indicating inflammation, but found no change in the anti-inflammatory marker CD206. Fat fibrosis was minimal, though DMM surgery showed a trend toward increased fibrosis. These data suggest that a high-fat diet metabolically 'charges' the IFP and that the combination of diet and a cartilage trigger brings the joint into a metabolic state of progressive OA.

Next, we investigated the involvement of the innate immune system in diet-induced OA. Male human C-reactive protein (hCRP) knock-in mice were fed a high-fat diet for 38 weeks (Chapter 5). We showed that hCRP-transgenic mice developed more severe OA compared to wild-type controls on the same diet. Since human CRP expression was the only variable, this implies that human CRP serves as an independent trigger to aggravate diet-induced OA. Increased recruitment of classical and non-classical monocytes may be a mechanism through which human CRP exacerbates OA.

Expanding on Chapters 3 and 5, Chapter 6 evaluated the contribution of elevated plasma cholesterol to diet-induced OA severity. Cholesterol is involved in both lipid metabolism and inflammation and is known to worsen OA. Novel, high-intensity cholesterol-lowering strategies were evaluated for their efficacy in halting the progression of pre-existing diet-induced OA. We observed negligible effects of therapeutic cholesterol-lowering on cartilage damage, suggesting that cholesterol-lowering treatments alone are likely ineffective in preventing the progression of pre-existing OA. The absence of inflammation in this model may explain the reduced effectiveness.

Chapter 7 contains the general discussion, placing the findings in the context of current knowledge and developments in the field. We discuss the implications and propose viewing the metabolic OA subtype as a whole-body condition.