Distribution and phenotype of murine rotavirus-specific B cells induced by intranasal immunization with 2/6 virus-like particles

Immunity to enteric viruses

Pathogenic enteric viruses are a major cause of morbidity and mortality, particularly among children in developing countries. The host response to enteric viruses occurs primarily within the mucosa, where the intestinal immune system must balance protection against pathogens with tissue protection and tolerance to harmless commensal bacteria and food. Here, we summarize current knowledge in natural immunity to enteric viruses, highlighting specialized features of the intestinal immune system. We further discuss how knowledge of intestinal anti-viral mechanisms can be translated into vaccine development with particular focus on immunization in the oral route. Research reveals that the intestine is a complex interface between enteric viruses and the host where environmental factors influence susceptibility and immunity to infection, while viral infections can have lasting implications for host health. A deeper mechanistic understanding of enteric anti-viral immunity with this broader context can ultimately lead to better vaccines for existing and emerging viruses.

Different profile and distribution of antigen specific T cells induced by intranasal and intrarectal immunization with rotavirus 2/6-VLP with and without LT-R192G

In this study, we compared both the profile and distribution of antigen specific primed T cells after intrarectal (IR) and intranasal (IN) immunization with rotavirus (RV) 2/6-VLP, alone or in the presence of LT-R192G, in order to highlight the differences between the two routes and the impact of the adjuvant. Adult BALB/c mice were immunized once with 2/6-VLP with or without adjuvant and the T cell response was analyzed in lymphoid tissues after in vitro restimulation with the antigen. IN, but not IR, immunization of mice with 2/6-VLP alone induced antigen-specific IL-10 and IL-17 secreting T cells. IL-10-, in contrast to IL-17-, secreting T cells did not migrate to the mesenteric lymph nodes (MLN) whereas they were detected in cervical lymph nodes (CLN) and spleen. With the IN route, the adjuvant allowed to complete this profile with the secretion of IL-2 and IL-4, increased IL-17 secretion and induced antigen specific CD4+CD25+Foxp3+ and Foxp3- T cells in all studied organs (CLN, spleen and MLN) but did not impact on IL-10 secreting T cells. With the IR route, the adjuvant induced IL-2 and IL-17 secretion but, in contrast to the IN route, did not allow IL-4 production. These results show that, for a same antigen, T cell priming not only depends on the presence of adjuvant but also on the mucosal route of administration. Moreover, they show a different dissemination of IL-10 secreting T cells compared to other subtypes.

Development of a Bacillus subtilis-based rotavirus vaccine

Bacillus subtilis vaccine strains engineered to express either group A bovine or murine rotavirus VP6 were tested in adult mice for their ability to induce immune responses and provide protection against rotavirus challenge. Mice were inoculated intranasally with spores or vegetative cells of the recombinant strains of B. subtilis. To enhance mucosal immunity, whole cholera toxin (CT) or a mutant form (R192G) of Escherichia coli heat-labile toxin (mLT) were included as adjuvants. To evaluate vaccine efficacy, the immunized mice were challenged orally with EDIM EW murine rotavirus and monitored daily for 7 days for virus shedding in feces. Mice immunized with either VP6 spore or VP6 vegetative cell vaccines raised serum anti-VP6 IgG enzyme-linked immunosorbent assay (ELISA) titers, whereas only the VP6 spore vaccines generated fecal anti-VP6 IgA ELISA titers. Mice in groups that were immunized with VP6 spore vaccines plus CT or mLT showed significant reductions in virus shedding, whereas the groups of mice immunized with VP6 vegetative cell vaccines showed no difference in virus shedding compared with mice immunized with control spores or cells. These results demonstrate that intranasal inoculation with B. subtilis spore-based rotavirus vaccines is effective in generating protective immunity against rotavirus challenge in mice.

Unexpected modulation of recall B and T cell responses after immunization with rotavirus-like particles in the presence of LT-R192G

LT-R192G, a mutant of the thermolabile enterotoxin of E. coli, is a potent adjuvant of immunization. Immune responses are generally analyzed at the end of protocols including at least 2 administrations, but rarely after a prime. To investigate this point, we compared B and T cell responses in mice after one and two intrarectal immunizations with 2/6 rotavirus-like particles (2/6-VLP) and LT-R192G. After a boost, we found, an unexpected lower B cell expansion measured by flow cytometry, despite a secondary antibody response. We then analyzed CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) and CD4(+)CD25(+)Foxp3(-) helper T cells after in vitro (re)stimulation of mesenteric lymph node cells with the antigen (2/6-VLP), the adjuvant (LT-R192G) or both. 2/6-VLP did not activate CD4(+)CD25(+)Foxp3(-) nor Foxp3(+) T cells from non-immunized and 2/6-VLP immunized mice, whereas they did activate both subsets from mice immunized with 2/6-VLP in the presence of adjuvant. LT-R192G dramatically decreased CD4(+)CD25(+)Foxp3(+) T cells from non-immunized and 2/6-VLP immunized mice but not from mice immunized with 2/6-VLP and adjuvant. Moreover, in this case, LT-R192G increased Foxp3 expression on CD4(+)CD25(+)Foxp3(+) cells, suggesting specific Treg activation during the recall. Finally, when both 2/6-VLP and LT-R192G were used for restimulation, LT-R192G clearly suppressed both 2/6-VLP-specific CD4(+)CD25(+)Foxp3(-) and Foxp3(+) T cells. All together, these results suggest that LT-R192G exerts different effects on CD4(+)CD25(+)Foxp3(+) T cells, depending on a first or a second contact. The unexpected immunomodulation observed during the recall should be considered in designing vaccination protocols.

Cholera-like enterotoxins and Regulatory T cells

Cholera toxin (CT) and the heat-labile enterotoxin of E. coli (LT), as well as their non toxic mutants, are potent mucosal adjuvants of immunization eliciting mucosal and systemic responses against unrelated co-administered antigens in experimental models and in humans (non toxic mutants). These enterotoxins are composed of two subunits, the A subunit, responsible for an ADP-ribosyl transferase activity and the B subunit, responsible for cell binding. Paradoxically, whereas the whole toxins have adjuvant properties, the B subunits of CT (CTB) and of LT (LTB) have been shown to induce antigen specific tolerance when administered mucosally with antigens in experimental models as well as, recently, in humans, making them an attractive strategy to prevent or treat autoimmune or allergic disorders. Immunomodulation is a complex process involving many cell types notably antigen presenting cells and regulatory T cells (Tregs). In this review, we focus on Treg cells and cholera-like enterotoxins and their non toxic derivates, with regard to subtype, in vivo/in vitro effects and possible role in the modulation of immune responses to coadministered antigens.

Intranasal immunization with vaccine vector Streptococcus gordonii elicits primed CD4+ and CD8+ T cells in the genital and intestinal tracts

Generation of primed T cells is crucial for the development of optimal vaccination strategies. Using a TCR-transgenic CD4(+) and CD8(+) T cell adoptive transfer model, we demonstrate that a single nasal immunization with recombinant Streptococcus gordonii induces antigen-specific primed T cells in lymph nodes draining the genital and intestinal tracts with about 80% of CD4(+) and 50% of CD8(+) proliferating cells. T cell clonal expansion was also observed in cervical lymph nodes, draining the immunization site, and in the spleen. The modulation of CD44 and CD45RB marker expression indicated that proliferating T cells were activated. Proliferation in distal mesenteric and iliac lymph nodes and in the spleen was observed 5 days after nasal immunization, while in draining cervical lymph nodes proliferation peaked already at day 3. The division profile of transgenic T cells observed in iliac and mesenteric lymph nodes was discontinuous, showing the lack of early cell divisions. The kinetics of T cell clonal expansion, the discontinuous division profile and the modulation of migration markers such as CD62L suggest that activated antigen-specific T cells disseminate from the immunization site to distal intestinal and genital tracts. These data demonstrate the efficacy of nasal immunization with recombinant S. gordonii in eliciting CD4(+) and CD8(+) T cell priming not only in draining sites, but also in the genital and intestinal tracts and in the spleen.

Distribution and phenotype of rotavirus-specific B cells induced during the antigen-driven primary response to 2/6 virus-like particles administered by the intrarectal and the intranasal routes

Selection of mucosal sites is an important step in mucosal vaccine development. The intrarectal (IR) route represents an alternative to the oral route of immunization; nevertheless, immune responses induced by this route are not well defined. Here, we studied the early primary B cell response (induction, homing, and phenotype) induced by IR immunization with rotavirus (RV)-2/6 virus-like particles (VLP). Using flow cytometry, we traced RV-specific B cells in different lymphoid tissues and analyzed the expression of alpha4beta7 and CCR9, which are important receptors for homing to the gut, as well as CD5, a marker expressed by B1-a cells, which are a major source of natural antibodies. We observed a massive, specific B cell response in rectal follicles, lumbar, and mesenteric lymph nodes but not in Peyer's patches or cervical lymph nodes. A minority of cells expressed alpha4beta7, suggesting a probable lack of migration to the gut, whereas CCR9 and CD5 were expressed by 30-50% and 30-75% of specific B cells, respectively. Then, we compared the intranasal route of immunization and observed similar B cell frequency and phenotype but in respiratory lymphoid tissues. These results confirm the high compartmentalization of B cell responses within the mucosal system. They show that CCR9 expression, conversely to alpha4beta7, is not restricted to B cells induced in the gut. Finally, an important part of the RV-specific B cell response induced at the mucosal level during the primary response to VLP is most likely a result of B1-a cells.

Induction of secretory immunity and memory at mucosal surfaces

Mucosal epithelia comprise an extensive vulnerable barrier which is reinforced by numerous innate defence mechanisms cooperating intimately with adaptive immunity. Local generation of secretory IgA (SIgA) constitutes the largest humoral immune system of the body. Secretory antibodies function both by performing antigen exclusion at mucosal surfaces and by virus and endotoxin neutralization within epithelial cells without causing tissue damage. SIgA is thus persistently containing commensal bacteria outside the epithelial barrier but can also target invasion of pathogens and penetration of harmful antigens. Resistance to toxin-producing bacteria such as Vibrio cholerae and enterotoxigenic Escherichia coli appears to depend largely on SIgA, and so does herd protection against horizontal faecal-oral spread of enteric pathogens under naïve or immunized conditions--with a substantial innate impact both on cross-reactivity and memory. Like natural infections, live mucosal vaccines or adequate combinations of non-replicating vaccines and mucosal adjuvants, give rise not only to SIgA antibodies but also to longstanding serum IgG and IgA responses. However, there is considerably disparity with regard to migration of memory/effector cells from mucosal inductive sites to secretory effector sites and systemic immune organs. Also, although immunological memory is generated after mucosal priming, this may be masked by a self-limiting response protecting the inductive lymphoid tissue in the gut. The intranasal route of vaccine application targeting nasopharynx-associated lymphoid tissue may be more advantageous for certain infections, but only if successful stimulation is achieved without the use of toxic adjuvants that might reach the central nervous system. The degree of protection obtained after mucosal vaccination ranges from reduction of symptoms to complete inhibition of re-infection. In this scenario, it is often difficult to determine the relative importance of SIgA versus serum antibodies, but infection models in knockout mice strongly support the notion that SIgA exerts a decisive role in protection and cross-protection against a variety of infectious agents. Nevertheless, relatively few mucosal vaccines have been approved for human use, and more basic work is needed in vaccine and adjuvant design, including particulate or live-vectored combinations.

Evaluation of Circulating Intestinally Committed Memory B Cells in Children Vaccinated with Attenuated Human Rotavirus Vaccine

In a double blind trial, 319 fully immunized children received two doses of either placebo or 10(6.7) focus-forming units of the attenuated RIX4414 human rotavirus (RV) vaccine ("all-in-one" formulation). Plasma RV-specific IgA (RV IgA), stool RV IgA, and circulating total and RV memory B cells (CD19+ IgD+/- CD27+) with an intestinal homing phenotype (alpha4beta7+ CCR9+/-) were measured, after the first and second doses, as potential correlates of protection. After the first and/or second dose, 54% of vaccinees and 13% of placebo recipients had plasma RV IgA. Before vaccination, most (95%) of the children (of both study groups) were breast-fed and had stool RV IgA (68.64%). Coproconversion (4-fold increase) after the first and/or second dose was observed in 32.7% of vaccinees and 17.4% of placebo recipients. No significant difference was seen when comparing the frequencies of any subset of memory B cells between vaccinees and placebo recipients. Statistically significant weak correlations were found between plasma RV IgA titers and coproconversion, and several subsets of memory B cells. The vaccine provided 74.8% protection (95% confidence interval, 30.93-92.62) against any RV gastroenteritis and 100% protection (95% confidence interval, 14.53-100) against severe RV gastroenteritis. When vaccinees and placebo recipients were considered together, a correlation was found between protection from disease and plasma RV IgA measured after dose 2 and RV memory (IgD- CD27+ alpha4beta7+ CCR9+) circulating B cells measured after dose 1. However, the correlation coefficients for both tests were low (<0.2), suggesting that other factors are important in explaining protection from disease.

Intrarectal immunization with rotavirus 2/6 virus-like particles induces an antirotavirus immune response localized in the intestinal mucosa and protects against rotavirus infection in mice

Rotavirus (RV) is the main etiological agent of severe gastroenteritis in infants, and vaccination seems the most effective way to control the disease. Recombinant rotavirus-like particles composed of the viral protein 6 (VP6) and VP2 (2/6-VLPs) have been reported to induce protective immunity in mice when administered by the intranasal (i.n.) route. In this study, we show that administration of 2/6-VLPs by the intrarectal (i.r.) route together with either cholera toxin (CT) or a CpG-containing oligodeoxynucleotide as the adjuvant protects adult mice against RV infection. Moreover, when CT is used, RV shedding in animals immunized by the i.r. route is even reduced in comparison with that in animals immunized by the i.n. route. Humoral and cellular immune responses induced by these immunization protocols were analyzed. We found that although i.r. immunization with 2/6-VLPs induces lower RV-specific immunoglobulin G (IgG) and IgA levels in serum, intestinal anti-RV IgA production is higher in mice immunized by the i.r. route. Cellular immune response has been evaluated by measuring cytokine production by spleen and Peyer's patch cells (PPs) after ex vivo restimulation with RV. Mice immunized by the i.n. and i.r. routes display higher gamma interferon production in spleen and PPs, respectively. In conclusion, we demonstrate that i.r. immunization with 2/6-VLPs protects against RV infection in mice and is more efficient than i.n. immunization in inducing an anti-RV immune response in intestinal mucosa.

Rectal immunization with rotavirus virus-like particles induces systemic and mucosal humoral immune responses and protects mice against rotavirus infection

To evaluate whether the rectal route of immunization may be used to provide appropriate protection against enteric pathogens such as rotaviruses (RV), we studied the antibody response and the protection induced by rectal immunization of mice with RV virus-like particles (VLP). For this purpose, 6-week-old BALBc mice were rectally immunized twice with RV 8-2/6/7-VLP derived from the bovine RV RF81 strain either alone or combined with various adjuvants including four toxins [cholera toxin (CT) and three attenuated Escherichia coli-derived heat-labile toxins (LTs), LT(R192G), LT(R72), and LT(K63)] and two Toll-like receptor-targeting adjuvants (CpG and resiquimod). Six weeks after the second immunization, mice were challenged with murine RV strain ECw. RV VLP administered alone were not immunogenic and did not protect mice against RV challenge. By contrast, RV VLP combined with any of the toxin adjuvants were immunogenic (mice developed significant titers of anti-RV immunoglobulin A [IgA] in both serum and feces and of anti-RV IgG in serum) and either efficiently induced complete protection of the mice (no detectable fecal virus shedding) or, for LT(K63), reduced the amount of fecal virus shedding after RV challenge. When combined with RV VLP, CpG and resiquimod failed to achieve protection, although CpG efficiently induced an antibody response to RV. These results support the consideration of the rectal route for the development of new immunization strategies against RV infection. Rectal delivery of a VLP-based vaccine might allow the use of adjuvants less toxic than, but as efficient as, CT.