S1PR1 and rheumatoid arthritis: However, our findings that EC S1PR1 signaling is protective in human RA are not congruent with a report showing elevated EC S1PR1 in RA synovium (72), a result that might be explained by the known increase in numbers of microvessels in hyperproliferative synovium rather than an increase in S1PR1 expression on individual ECs.