Reactive arthritis-associated bacteria can stimulate lymphocyte proliferation in non-exposed individuals and newborns.

In reactive arthritis (ReA) a specific T cell response to the triggering bacterial antigen is present in the synovial fluid, while in paired peripheral blood T cells the response is markedly reduced. The proliferative response to ReA-associated bacteria in the peripheral blood of ReA patients was compared with that seen in the blood of healthy adults, who denied exposure to these microbes, and in the umbilical cord blood of newborns, who have clearly not been exposed to bacterial antigen. Peripheral blood mononuclear cells (PBMC) from non-exposed adults and those from umbilical cord blood proliferated to ReA-associated bacteria, whilst little response was seen in ReA PBMC. The response was MHC class II-restricted, required processing of the bacterial antigen, was seen in both CD45RO+ and CD45RA+ subsets, and was not oligoclonal. These T cell responses are similar to those previously demonstrated in non-exposed individuals to malaria, leishmania and trypanosoma antigen, and may reflect the existence of 'natural' T cell immunity to ReA-associated bacteria. The lack of such responses in ReA peripheral blood may suggest that such 'natural' responses may restrict the dissemination or progression of infection.