Search for immunosuppressive substances produced in the culture of enteric bacteria isolated from marine fish

Culture supernatants of enteric bacteria isolated from the gastrointestinal tracts of two marine fishes were screened for the production of immunosuppressive substances. At least 50% of the 60 bacterial isolates tested were found to produce immunosuppressants that significantly inhibited the induction of cytotoxic T cells (CTL) in vitro. The strains that secreted the highest suppressive activity was identified as Aeromonas caviae. The immunosuppressive substance purified from the A. caviae culture supernatant was a polysaccharide with a molecular weight of about 70 kDa that contained mannose : glucose (4:1). This polysaccharide did not show nonspecific cytotoxicity, and was found to inhibit the proliferation and differentiation of CD8-positive CTL.