Modulation of Specific Immune Responses The IgA Anti Rotavirus Response

Little information is available regarding the role of intestinal microbiota composition on the modulation of the specific sIgA Ab response against enteropathogens. Indeed, it can be assumed that, according to the composition of the digestive microbiota and the presence, or not, of some bacteria in the dominant microbiota, the specific immune responses might be different.

This fact is of particular importance in babies where the poorly diversified intestinal microbiota is strongly influenced by the type of milk. Indeed, it is well known that breastfed babies are more resistant to enteric infections than formula-fed babies (69,70). Human breast milk contains abundant bioactive components that may provide direct protective effects to infants against enteric pathogens (71), but breast-feeding also influences the intestinal microbiota composition enhancing Bifidobacterium development. To test the influence of the intestinal microbiota on the modulation of a specific intestinal sIgA-Ab response, a sIgA anti-rotavirus response was established in a mouse model. This involved an original model of adult gnotobiotic mice colonized with the fecal microbiota of a breast- or a bottle-fed infant and then orally inoculated with a heterologous simian rotavirus strain SA-11. As previously described, the adult mouse model described here excluded breast milk effects and the possible immaturity of the neonate immune system [(72) and manuscript in preparation].

Bacterial strains found in the dominant fecal microbiota of a breast- or formula-fed baby were isolated and inoculated in the digestive tract of the gnotobiotic mice. They established in a similar manner as in babies. "Breast-fed mice" were colonized with Bifidobacterium, Escherichia coli and Streptococcus, while only two Gram-negative bacteria, E. coli and Bacteroides, colonized the digestive tract of "formula-fed" mice. The two groups of gnotobiotic mice were similar in all respects except for the intestinal microbiota and especially by the presence or absence of Bifidobacterium. They were orally inoculated with rotavirus 3 weeks after bacterial colonization to allow the bacteria time to affect the immune system of the host. The kinetics of sIgA anti-rotavirus Ab responses were measured in feces by enzyme linked immunosorbent assay (ELISA) over a one month period of time and at sacrifice, numbers of sIgA-anti-rotavirus secreting cells were evaluated in the small intestine by solid phase enzyme-linked immunospot (ELISPOT) assay.

Kinetics of the sIgA anti-rotavirus response were similar in the two groups of gnotobiotic mice, but the maximal level, that was reached 20 days after viral inoculation, was approximately 4-fold higher in "breast-fed" than in "formula-fed" gnotobiotic mice (Fig. 3). The same difference was measured for the sIgA-anti-rotavirus secreting cell numbers. To assess the respective immunomodulatory role of two bacteria present in the baby's intestine, Bifidobacterium bifidum (Gram-positive bacteria) and E. coli

Gnotobiotic mice : « Breast-fed baby »

Gnotobiotic mice « Formula-fed baby » (Escherichia coli, Bacteroides)

(Bifidobacterium, Escherichia coli, Streptococcus)

Oral Inoculation with rotavirus iL

Three weeks later; level of anti-rotavirus sIgA antibodies in feces

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