IL-17 has also been found in joint tissue in higher quantity in RA than in osteoarthritis.
These PAMP PRR interactions are probably not sufficient to explain activation of FLSs in RA and it is evident that additional factors, such as genetic polymorphisms, play an important role, because bacterial cell wall components, such as peptidoglycan, have also been found in joint tissues from OA patients but do not induce the inflammatory response occurring in RA.
Some TLRs such as TLR2, TLR3, and TLR4 can be activated by endogenous 'danger' molecules associated with inflammation and tissue destruction, and many of these molecules have been found in joints and serum of patients with RA and show a positive correlation with disease activity scores [ 4, 5].
OCP crystals, the most potent inflammation-inducing BCP crystals [ 33, 34], are found in joint fluids [ 35].
However, when these drugs are administered topically in proximity to a joint (for example, on top of the knee), the drug can be found in joint tissues such as the synovium and cartilage [ 58].
The number of capillaries in active tissue, such as muscle, liver, kidney, and lungs, is greater than the number in tendon or ligament; in addition, the cornea of the eye, epidermis, and hyaline cartilage (semitransparent cartilage such as is found in joints) are devoid of capillaries.
Examples of non-neuropathic conditions are light chain amyloidosis and hemodialysis-related amyloidosis, where deposits are found in joints, skeletal tissue, heart, kidney, etc.
As in RA, a hyperplastic synovium staining positively for CD3+ and CD68+ infiltrated cells can be found in joints lacking macroscopic signs of arthritis [ 30].
Additional analyses also showed that the genetic correlation between effusions and palpatory joint findings were very high with an estimated value of 1.00 (s.e. 0.01) and a phenotypic correlation of 0.92, thus most effusions are found in joints and most joint findings are effusions, and these traits could be considered to have the same genetic background.
In earlier studies [ 31], and in control experiments (data not shown), using fluorescein-labeled or isotope-labeled donor lymphocytes, only very few cells were found in joints during the first week of transfer, and a second cell transfer was needed to induce a significant influx of lymphocytes into the joints and cause subsequent inflammation.
While the endocannabinoid arachidonylethanolamine (anandamide, AEA) and 2-archidonyl glycerol (2-AG) are absent from healthy joints, where related N-acylethanolamines oleoylethanolamine (OEA) and palmitoylethanolamine (PEA) are abundant, both 2-AG and AEA were found in joints of arthritic and osteoarthritic patients [ 6].
