We developed in vitro models to study the human systemic and intestinal RV immune response. The evaluation of systemic antigen presenting cells (APC)
showed that monocyte derived dendritic cells (moDC) in contact with infectious RV are able to stimulate a strong CD4 Th1 allogeneic response (Narváez et al., 2005). Besides, circulating plasmacytoid dendritic cells are necessary to stimulate RV-specific memory T cells to produce IFN-γ (Mesa et al., 2007). Because moDC generally reflect the function of circulating peripheral blood myeloid dendritic cells, these findings predict that an important systemic Th1 response against RV should be generated under the viremic phase of infection. Since, as previously described, this is not the case, our results suggest that either the systemic antigen/virus that circulates during an acute infection with RV is presented to T cells in a tolerogenic context or that the amount of virus present systemically is unsuited for T-cell immunity induction.
The tolerogenic gut environment (
Lamichhane et al., 2014) may substantially influence the T-cell response against RV. For the development of an in vitro model of human intestinal immune response against RV, we initially cultivated polarized Caco-2 cells in transwells and identified the “danger signals” released by cells infected by RV (
Rodríguez et al., 2009). After infection, Caco-2 cells released IL-8, PGE
2, small quantities of TGF-β1, and the constitutive and inducible heat shock proteins HSC70 and HSP70, which are known to induce a noninflammatory (non-Th-1) immune response (
Rodríguez et al., 2009). Furthermore, HSC70, HSP70, and TGF-β1 were released, in part, associated with membrane vesicles (MV) obtained from filtrated Caco-2 supernatants concentrated by ultracentrifugation (
Barreto et al., 2010). These MV were heterogeneous, with characteristics of exosomes and probably also of apoptotic bodies, and had immunomodulatory functions: MV from RV-infected cells induced death and inhibited proliferation of polyclonally stimulated CD4 T cells, and these effects were in part due to TGF-β (
Barreto et al., 2010).
We next studied the effect of these intestinal immunomodulators in relation to the interaction of RV with moDC (
Rodriguez et al., 2012): moDC treated with supernatants from RV-infected Caco-2 cells promoted a significantly lower Th1 response, in comparison with those treated with purified RV. Moreover, TGF-β, unlike thymic stromal lymphopoietin (TSLP), was an importat mediator of this modulation, suggesting that TGF-β could be an immune evasion mechanism (
Rodriguez et al., 2012). In agreement with this hypothesis, in PBMC from healthy adults the inhibition of the TGF-β signaling pathway increased the frequency of RV-specific CD4 T cells that produce IFN-γ (
Mesa et al., 2010). However, this inhibition was undetected in children, suggesting that RV-specific CD4 T cells could be modulated by other tolerogenic mechanisms, such as anergy.
We used three anergy inhibitors to assess the hypothesis of the presence of circulating anergic T cells in the response against RV: after stimulation of PBMC from healthy adults with RV in the presence of IL-2—unlike IL-12 or R59949 (a pharmacological diacyl- glycerol kinase alpha inhibitor)—increased frequencies of RV-specific CD4 and CD8 T cells producing cytokines were identified (
Parra et al., 2014b). This finding depicts a poor functional T-cell profile that may be partially reversed in vitro by the addition of rIL-2.
The role of regulatory T cells (Treg) in RV infection has been investigated in few reports. In RV-infected mice, the numbers of FoxP3
+ Treg cells are increased, but RV clearance or Abs levels are unsignificantly modified in their absence (
Miller et al., 2014). In PBMC from healthy adults the depletion of CD25
+ T cells (probably containing Treg cells) increases the frequency of RV-specific CD4 T cells that produce IFN-γ, suggesting that, at least systemically, Treg may modulate the function of RV-specific CD4 T cells (
Mesa et al., 2010).