Bromelain

Bromelain is a digestive enzyme found in pineapples. Digestive enzymes break down proteins into smaller parts. One review article summarizes the findings of many studies:

The use of bromelain for the treatment of osteoarthritis looks promising. However, a number of methodological caveats indicate that further studies are warranted, in particular phase II
clinical trials to identify the optimum dosage, followed by a definitive randomised placebo-controlled trial to confirm its efficacy in the treatment of osteoarthritis.

It is not known how bromelain changes the immune response but many mechanisms have been postulated for bromelain.

Experimental evidence suggests that bromelain’s action as an anti-inflammatory is mediated via the following factors: (i) by increasing serum fibrinolytic activity (13), reducing plasma fibrinogen levels (14) and decreasing bradykinin levels (which results in reduced vascular permeability) and hence reducing oedema and pain (15); (ii) by mediating prostaglandin levels (by decreasing levels of PGE2 and thromboxane A2); and (iii) through modulation of certain immune cell surface adhesion molecules (16–20), which play a role in the pathogenesis of arthritis (21). However, many of these studies are of poor quality and further data is needed to clarify definitive mechanisms of its action.

https://downloads.hindawi.com/journals/ecam/2004/373074.pdf