Efficient induction of mucosal immunity usually employs nasal or oral vaccination while parenteral immunization generally is ineffective at generating mucosal immune responses. This relates to the unique ability of resident mucosal dendritic cells (DC) to induce IgA switching and to imprint mucosa-specific homing receptors on lymphocytes. Based on the well-established plasticity of the DC system, this study sought to investigate whether peripheral DC could be modulated toward “mucosa-type” DC by treatment with immunomodulatory, and therefore potentially adjuvant-like, factors. In this study, we show that monocyte-derived DCs pretreated with the vitamin A derivative all-trans retinoic acid (RA) indeed acquired several attributes characteristic of mucosal DC: secretion of TGF-β and IL-6 and the capacity to augment mucosal homing receptor expression and IgA responses in cocultured lymphocytes. Addition of a TGF-β-neutralizing Ab to cocultures significantly inhibited α4β7 integrin, but not CCR9 mRNA expression by the lymphocytes. Both α4β7 integrin and CCR9 mRNA expression, but not IgA production, were suppressed in the presence of a RA receptor antagonist. None of the observed effects on the lymphocytes were influenced by citral, a retinal dehydrogenase inhibitor, arguing against a role for de novo-synthesized RA. Collectively, our findings identified a novel role for RA as a mucosal immune modulator targeting DC. Our results further demonstrate that DC can act as efficient carriers of RA at least in vitro. Consequently, RA targeting of DC shows potential for promoting vaccine-induced mucosal immune responses via a parenteral route of immunization.

Infection of pigs with classical swine fever virus (CSFV), a member of the Flaviviridae family, causes a severe leukopenia, particularly notable with the lymphocytes. The goal of this study was to analyze mechanisms behind this CSFV-induced lymphopenia. To this end, the kinetics of leukocyte depletion, the appearance of apoptotic cells, and virus infection of leukocytes after infection of pigs with the virulent CSFV strain Brescia were analyzed. Depletion of B and T lymphocytes was noted as early as 1 day postinfection (p.i.). Circulating viable lymphocytes with reduced mitochondrial transmembrane potential—a particular early marker for apoptosis—were also detectable as early as 1 day p.i. When isolated peripheral blood mononuclear cells were cultured for 6 h, significantly more sub-G1 cells with reduced DNA content were detected among the lymphocytes from CSFV-infected animals, again as early as 1 to 3 days p.i. The first time virus was first found in the plasma, as well as infection of leukocytes, was 3 days p.i. However, throughout the observation time of 1 week, <3% of the circulating leukocytes and no lymphocytes contained virus or viral antigen. Further analysis of the T lymphocytes from infected animals demonstrated an increase in CD49d, major histocompatibility complex class II, and Fas expression. An increased susceptibility to apoptosis in vitro was also observed, particularly after addition of concanavalin A as well as apoptosis-inducing anti-Fas antibody to the cultures. Taken together, these results imply that activation-induced programmed cell death was the mechanism behind lymphopenia during classical swine fever.

Abstract:The purpose of this study was the discrim-

Pseudorabies virus (PRV; suid herpesvirus 1) infection causes heavy economic losses in the pig industry. Therefore, vaccination with live attenuated viruses is practiced in many countries. This vaccination was demonstrated to induce extrathymic virus-specific memory CD4+CD8+ T lymphocytes. Due to their major histocompatibility complex (MHC) class II-restricted proliferation, it is generally believed that these T lymphocytes function as memory T-helper cells. To directly prove this hypothesis, 15-amino-acid, overlapping peptides of the viral glycoprotein gC were used for screening in proliferation assays with peripheral blood mononuclear cells of vaccinated d/d haplotype inbred pigs. In these experiments, two naturally processed T-cell epitopes (T1 and T2) which are MHC class II restricted were identified. It was shown that extrathymic CD4+CD8+ T cells are the T-lymphocyte subpopulation that responds to epitope T2. In addition, we were able to show that cytokine secretion can be induced in these T cells through recall with inactivated PRV and demonstrated that activated PRV-primed CD4+CD8+ T cells are able to induce PRV-specific immunoglobulin synthesis by PRV-primed, resting B cells. Taken together, these results demonstrate that the glycoprotein gC takes part in the priming of humoral anti-PRV memory responses. The experiments identified the first T-cell epitopes so far known to induce the generation of virus-specific CD4+CD8+ memory T lymphocytes and showed that CD4+CD8+ T cells are memory T-helper cells. Therefore, this study describes the generation of virus-specific CD4+CD8+ T cells, which is observed during vaccination, as a part of the potent humoral anti-PRV memory response induced by the vaccine.

The induction of immune responses requires critical interaction between innate parts of the immune system, which respond rapidly and in a relatively nonspecific manner, and other specific parts, which recognize particular epitopes on an antigen. A critical element in this interaction is the role played by dendritic cells (DCs), which represent "professional antigen-presenting cells." DCs endocytose and process antigen to peptide presented on the cell surface in association with major histocompatibility complex (MHC) molecules. This presentation results in interaction with and stimulation of helper T (Th) lymphocytes, which recognize peptide in association with either MHC class II or cytotoxic T (Tc) lymphocytes, which recognize peptide in association with MHC class I. Stimulation of Th lymphocytes produces the growth and differentiation factors (cytokines) essential for the B lymphocytes that have responded to a more intact form of the antigen and that differentiate into antibody-producing cells. The precise interaction between the cells depends on cognate ligand-receptor recognition between the B and Th lymphocytes. DCs also play a direct role with the stimulation of the B lymphocytes. It appears that DC can deliver antigen to the B lymphocytes in a more intact form than the processed form essential for stimulating T lymphocytes, and can release cytokines that assist the differentiation of the B lymphocytes into antibody-producing cells. This close relationship among the three cell types and the cytokines that are produced ensures the precise control and regulation necessary for immune response development.