The reduced efficiency of the mammalian immune system with aging increases host susceptibility to infectious and autoimmune diseases. However, the mechanisms responsible for these pathologic changes are not well understood. In this study, we demonstrate that the bone marrow, blood, and secondary lymphoid organs of healthy aged mice possess increased numbers of immature myeloid cells that are phenotypically similar to myeloid-derived suppressor cells found in lymphoid organs of mice with progressive tumors and other pathologic conditions associated with chronic inflammation. These cells are characterized by the presence of Gr1 and CD11b markers on their surfaces. Gr1+CD11b+ cells isolated from aged mice possess an ability to suppress T cell proliferation/activation and produce heightened levels of proinflammatory cytokines, both constitutively and upon activation, including IL-12, which promotes an excessive production of IFN-γ. IFN-γ priming is essential for excessive proinflammatory cytokine production and the suppressive activities by Gr1+CD11b+ cells from aged mice. These cells suppress T cell proliferation through an NO-dependent mechanism, as depletion of splenic Gr1+ cells reduces NO levels and restores T cell proliferation. Insights into mechanisms responsible for the proinflammatory and immune suppressive activities of Gr1+CD11b+ cells from aged mice have uncovered a defective PI3K–Akt signaling pathway, leading to a reduced Akt-dependent inactivation of GSK3β. Our data demonstrate that abnormal activities of the Gr1+CD11b+ myeloid cell population from aged mice could play a significant role in the mechanisms responsible for immune senescence.

The addition of monophosphoryl lipid A, a minimally toxic derivative of LPS, to nonmucosally administered vaccines induced both systemic and mucosal immune responses to coadministered Ags. This was dependent on an up-regulated expression of 1α-hydroxylase (CYP27B1, 1αOHase), the enzyme that converts 25-hydroxycholecalciferol, a circulating inactive metabolite of vitamin D3, into 1,25(OH)2D3 (calcitriol). In response to locally produced calcitriol, myeloid dendritic cells (DCs) migrated from cutaneous vaccination sites into multiple secondary lymphoid organs, including classical inductive sites of mucosal immunity, where they effectively stimulated B and T cell immune responses. The endogenous production of calcitriol by monophosphoryl lipid A-stimulated DCs appeared to be Toll-IL-1R domain-containing adapter-inducing IFN-β-dependent, mediated through a type 1 IFN-induced expression of 1αOHase. Responsiveness to calcitriol was essential to promote the trafficking of mobilized DCs to nondraining lymphoid organs. Collectively, these studies help to expand our understanding of the physiologically important roles played by locally metabolized vitamin D3 in the initiation and diversification of adaptive immune responses. The influences of locally produced calcitriol on the migration of activated DCs from sites of vaccination/infection into both draining and nondraining lymphoid organs create a condition whereby Ag-responsive B and T cells residing in multiple lymphoid organs are able to simultaneously engage in the induction of adaptive immune responses to peripherally administered Ags as if they were responding to an infection of peripheral or mucosal tissues they were designed to protect.

Cathelicidin production by human myeloid cells stimulated through toll like receptor (TLR) 2/1, the migration of human CD8 + T cells to inflamed skin sites, and the ability of murine dendritic cells (DCs) to migrate from skin sites of vaccination to mucosal lymphoid organs all occur via calcitrioldependent mechanisms. Herein, we report that murine DCs exposed to TLR3/TLR4 ligands upregulate their expression of 1α-hydroxylase, the enzyme that converts circulating 25(OH)D 3 to calcitriol, the active form of vitamin D3. TLR3/TLR4 ligands injected subcutaneously affect DC migration in vivo, allowing their trafficking to both draining and non-draining systemic and mucosal lymphoid organs. Subcutaneously delivered vaccines containing TLR3/TLR4 ligands and antigen stimulate the induction of both systemic and mucosal immune responses. Vaccines containing TLR9 ligands fail to stimulate 1α-hydroxylase protein expression, are incapable of redirecting DC migration into Peyer's patches and do not induce mucosal immune responses. These findings support a hypothesis that active metabolites of vitamin D3 produced locally are able to affect various aspects of innate and acquired immune responses. Keywords dendritic cells; TLR ligands; mucosal immune response; calcitriol IntroductionIn mammals there are at least 12 members of the toll like receptor (TLR) family. These receptors not only recognize a number of specific components conserved among microorganisms, but are also capable of recognizing specific defensins as well as fragments of extracellular matrix proteins [1][2][3][4][5][6][7]. Since, the activation of macrophages or dendritic cells (DCs) through one or more of their TLRs enhances innate immunity and can modulate the subsequent development of antigen-specific adaptive immunity, some TLR ligands have been used as adjuvants in vaccine formulations administered parenterally to augment systemic immune responses [8][9][10][11]. Specific TLR ligands have been reported to promote the induction of both systemic and mucosal immune responses when co-administered with antigen intranasally [9][10][11][12].*Corresponding author. Address: Department of Pathology, University of Utah Medical School, 30 North 1900 East, Salt Lake City, UT 84132, USA; Phone: 1-801-585-1522; Fax: 1-801-581-8946; e-mail: elena.enioutina@path.utah.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptVaccine. Author manuscript; available in PMC 2009 January 30. Published in final edited form as:Vaccine. We have previously demonstrated that the subcutaneous or intradermal imm...

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Immunity conferred by conventional vaccines is restricted to a narrow range of closely related strains, highlighting the unmet medical need for the development of vaccines that elicit protection against multiple pathogenic serotypes. Here we show that a Salmonella bivalent vaccine comprised of strains that lack and overproduce DNA adenine methylase (Dam) conferred cross-protective immunity to salmonella clinical isolates of human and animal origin. Protective immunity directly correlated with increased levels of cross-reactive opsonizing antibodies and memory T cells and a diminished expansion of myeloid-derived suppressor cells (MDSCs) that are responsible for the immune suppression linked to several conditions of host stress, including chronic microbial infections, traumatic insults, and many forms of cancer. Further, aged mice contained increased numbers of MDSCs and were more susceptible to Salmonella infection than young mice, suggesting a role for these cells in the immune declines associated with the natural aging process. These data suggest that interventions capable of reducing MDSC presence and activities may allow corresponding increases in B-and T-cell stimulation and benefit the ability of immunologically diverse populations to be effectively vaccinated as well as reducing the risk of susceptible individuals to infectious disease.