Segmented filamentous bacteria are potent stimuli of a physiologically normal state of the murine gut mucosal immune system.

Segmented filamentous bacteria (SFB) are autochthonous bacteria inhabiting the intestinal tracts of many species, including humans. We studied the effect of SFB on the mucosal immune system by monoassociating formerly germfree C3H/HeN mice with SFB. At various time points during 190 days of colonization, fragment cultures of small intestine and Peyer's patches (PP) were analyzed for total immunoglobulin A (IgA) and SFB-specific IgA production. Also, phenotypic changes indicating germinal center reactions (GCRs) and the activation of CD4(+) T cells in PP were determined by using fluorescence-activated cell sorter analyses. A second group of SFB-monoassociated mice was colonized with a gram-negative commensal, Morganella morganii, to determine if the mucosal immune system was again stimulated and to evaluate the effect of prior colonization with SFB on the ability of M. morganii to translocate to the spleen and mesenteric lymph nodes. We found that SFB stimulated GCRs in PP from day 6 after monoassociation, that GCRs only gradually waned over the entire length of colonization, that natural IgA production was increased to levels 24 to 63% of that of conventionally reared mice, and that SFB-specific IgA was produced but accounted for less than 1.4% of total IgA. Also, the proportion of CD4(+), CD45RBlow T cells, indicative of activated cells, gradually increased in the PP to the level found in conventionally reared mice. Secondary colonization with M. morganii was able to stimulate GCRs anew, leading to a specific IgA antibody response. Previous stimulation of mucosal immunity by SFB did not prevent the translocation of M. morganii in the double-colonized mice. Our findings generally indicate that SFB are one of the single most potent microbial stimuli of the gut mucosal immune system.

Barren female Strongyloides stercoralis from occult chronic infections are rejuvenated by transfer to parasite-naive recipient hosts and give rise to an autoinfective burst.

It is widely assumed that barren Strongyloides stercoralis occurring in chronically infected carriers can become fecund when immunity wanes. Evidence for this involves corticosteroid treatment of hosts harboring occult infections that subsequently return to patency. However, nematodes have ecdysteroid receptors, and it has been suggested that corticosteroids act directly on the parasite, inducing autoinfective development, rather than indirectly by suppressing host immunity. To test these competing concepts, barren females were recovered from donor dogs when the dogs' fecal examinations turned negative. Groups of 100 active barren worms were surgically transplanted into the small intestines of each of 6 naive canine recipients. Three were examined at necropsy at 4-5 days postinfection (PI), before autoinfection could amplify the number of successfully transferred parasites. The remaining recipients were examined 21-22 days PI when, if autoinfection had occurred, the worm populations should have increased. At 4-5 days, gravid worms occurred in each of the recipients (19 +/- 6 worms/dog). By 21-22 days, a remarkable population increase had occurred (522.6 +/- 296 worms/dog). Worms from chronically infected donors were stunted, and electron microscopy revealed damage to the intestine and ovaries. Successfully transplanted worms recovered at days 4-5 PI were ovigerous and less stunted and showed repair of intestinal and ovarian tissues.