Dendritic cells (DC), known as professional antigen-presenting cells, are involved in the regulation of mucosal immune responses. In peripheral tissues, contact with antigen induces a maturation process and the migration of DC to secondary lymphoid organs, where they induce the differentiation of na¨ıve T cells [1, 2]. The induced profile (i.e., Th1, Th2, or regulatory) depends on the nature of the signals received and delivered by DC . In atopic patients, DC play a pivotal role in the development and the maintenance of allergic diseases characterized by a predominant Th2 profile [3, 4].
The increased risk of allergic diseases observed in industrialized countries was suggested to be related to western life style characterized by a reduced overall exposure to microbial stimulation in infancy [5–7]. Postnatal exposure to microbial antigens elicits preferentially Th1 or T regulatory responses, which have been suggested to counterbalance Th2- polarized cytokine production in neonates. Insufficient early microbial exposuremay favor the persistence of Th2-type cytokines production allowing the potential development of allergy. The intestine is one of the major immune organs of the body where DC can encounter bacteria [8, 9]. The predominant sites of antigen sampling are the Peyer patches, where intestinal DC interact directly with luminal bacteria by passing their dendrites between epithelial tight junctions or after transport of bacteria through M cells [10, 11].
Some commensal organisms are used as probiotics, that is, live microbial food ingredients with health-promoting properties . Recent studies suggest that healthy gut microbiota may have a crucial role for the maturation of the immune system to nonallergic mode. The intestinal flora of allergic children is less often colonized with lactobacilli in comparison with nonallergic children . A clinical study demonstrates that perinatal administration of probiotic bacteria (Lactobacillus GG) halved the later development of atopic eczema in high-risk children [14, 15]. This effect may be due to the anti-inflammatory properties of probiotic bacteria. Consumption of Lactobacillus GG by children with atopic dermatitis has been reported to enhance the generation of interleukin (IL)-10 in serum . Moreover oral feeding of mouse with heat-killed Lactobacillus casei (strain shirota) inhibited specific IgE production . As DC inhabiting the gut, mucosa are constantly in close vicinity to microorganisms, the intestinal flora may exert regulatory effects through DC modulation. Lactobacilli were reported to regulatemouseDC surfacemolecule expression and cytokine production .We previously demonstrated that three different LAB strains were able to inhibit the secretion of Th2 cytokines by peripheral blood mononuclear cells from allergic donors .Moreover, we confirmed for one of the strain (L. plantarum) the inhibition of the Th2 response induced by Der p 1-pulsed DC from allergic patients in vitro. This was associated with increased IL-12 secretion by DC, reduced Th2 cytokine production (IL-4, IL-5), and increased production of Th1 cytokine (interferon (IFN)-γ) by autologous T cells .
The presence of LAB in the intestinal environment might be an important factor required to avoid the development of the allergy-associated Th2 response. In view of the critical importance of DC in the regulation of the immune response, we analyzed the reactivity of human monocytederived DC to the LAB L. casei (ATCC 393) by using a welldefined experimental model of intestinal epithelium. Thus the effect of a L. casei strain on activation patterns of DC from allergic and healthy donors was investigated and the impact on T cell-dependent cytokine production was evaluated.