Toll-like receptor ligands directly promote activated CD4+ T cell survival

Up-regulation of cell surface Toll-like receptor 4-MD2 expression on dendritic epidermal T cells after the emigration from epidermis during cutaneous inflammation

BACKGROUND

Mouse epidermis contains a population of gammadelta T cells, termed dendritic epidermal T cells (DETCs), which uniformly express the invariant Vgamma3 T cell receptor. Certain DETC lines were reported to respond to Gram-negative bacteria in the presence of immobilized anti-CD3 monoclonal antibody or to lipopolysaccharide (LPS) in the presence of B cell lines.

OBJECTIVE

To determine whether DETCs express the primary signaling receptor for LPS, Toll-like receptor (TLR) 4-MD2.

METHODS

We analyzed expression of TLR4-MD2 in three independent DETC lines as well as in freshly isolated DETCs.

RESULTS

All DETC lines expressed TLR4 and MD2 transcripts and TLR4-MD2 protein complex intracellularly, but none expressed TLR4-MD2 on the cell surface. By immunoblotting, only the immature form of TLR4 protein was detected in the DETC lines. The DETC lines did not respond to LPS even in the presence of immobilized anti-CD3 monoclonal antibody. Freshly isolated DETCs and their fetal thymic precursors also lacked cell surface expression of TLR4-MD2, but a small subpopulation of dermal Vgamma3 T cells isolated from croton oil-painted skin expressed TLR4-MD2 on the cell surface. Similarly, Vgamma3 T cells emigrated from organ-cultured epidermis expressed cell surface TLR4-MD2.

CONCLUSIONS

These results demonstrate that DETCs do not constitutively express cell surface TLR4-MD2, but TLR4-MD2 expression may be up-regulated when DETCs emigrate from epidermis during cutaneous inflammation.

Toll-like receptor 4 or 2 agonists decrease allergic inflammation

Toll-like receptors (TLRs) recognize highly conserved microbial molecular patterns, such as found in endotoxin. This study tested whether TLR4 and TLR2 stimulation in vivo would modulate subsequent adaptive (allergic) immune responses. We analyzed the effects of pulmonary administration of a TLR4 agonist, lipid A (LpA), and two TLR2 agonists, peptidoglycan (Ppg) and PamCys, in a murine model of allergic inflammation. The TLR agonists were administered during allergen sensitization or challenge. Both TLR agonists decreased the allergen-induced pulmonary recruitment of eosinophils when administered at sensitization or challenge. When given before sensitization, the TLR4 and TLR2 agonists decreased additional allergen-induced parameters of inflammation (pulmonary eosinophilia, bronchoalveolar lavage IL-13, total serum IgE, and airway hyperresponsiveness). Interestingly, TLR4 and TLR2 agonists decreased the number of CD4+ cells in the lung. Also, at the site of local allergen stimulation, the draining thoracic lymph nodes, allergen-induced lymphocyte proliferation, and IL-13 secretion were decreased by administration of LpA and Ppg. These data provide a distinct example of the modulation of adaptive (allergic) responses by non-antigen-dependent stimuli. Our findings also demonstrate that both TLR4 and TLR2 agonists decrease allergic responses, supporting the concept that exposure to bacterial components under defined conditions may protect against allergic disease.

Understanding asthma pathogenesis: linking innate and adaptive immunity

PURPOSE OF REVIEW

Treatment and even prevention of allergic asthma will require a detailed understanding of disease pathogenesis and in particular identification of factors that govern T-helper type 2 (Th2) immunity. This review defines the priming and differentiation steps necessary to develop antiallergen Th2 immunity and highlights recently identified stimuli that satisfy these requirements.

RECENT FINDINGS

Striking discoveries in innate immunity have advanced our understanding of how adaptive immune responses are initiated, yet only recently have these principles been applied to allergic disease. Signaling through certain innate immune receptors, the toll-like receptors (TLR) have been shown to modulate Th2-mediated disease in animal models. The dendritic cell has emerged as the central player in the intricate interplay between the adaptive and innate systems of immunity. Recent studies have also uncovered alternative pathways of initiating allergen sensitization that depend entirely on adaptive, rather than innate immune, triggers.

SUMMARY

The adaptive immune system cannot initiate a response without the "permission" of the innate immune system, and this holds true for Th2 responses to aeroallergens, although induction of Th2 immunity in response to TLR signaling varies with the type and dose of TLR ligand. However, under conditions of ongoing Th2 inflammation, the adaptive immune system can act as its own adjuvant and provide the necessary activating signals to initiate an immune response to foreign protein antigens. This may be the mechanism underlying the clinically observed phenomenon of polysensitization in atopic patients and provides another therapeutic target in asthma.

What's new--what's hot in basic science: American Transplant Congress 2004

The field of transplantation biology has, in the past year, given rise to several improved models explaining the in vivo phenomena of allograft rejection and acceptance. Although T-cells remain central participants in allorecognition, innate immune cells are increasingly recognized as critical. There is also growing acceptance that T-cell responses can vary widely not only based on their repertoire, but also on their immune experience in general, and their participation in homeostatic proliferation. Additional signaling pathways and molecules, such as the Janus kinase pathway and vascular endothelial growth factor have been added to those recognized as important in pharmacologic immunosuppression, and the concepts of regulation, once polarized between CD8+ and CD4+ camps, have begun to converge upon a recognition that there are many phenotypes of regulatory cells. In addition to improved basic science, several translational fields have accelerated in the past 12 months. Promising and clinically applicable islet transplant regimens have been developed and look appropriate for near-term clinical trials. Significant progress in discordant xenotransplantation is also apparent. This manuscript will review the past year in transplantation science as reported at the 2004 American Transplant Congress.

Toll-like receptors and other links between innate and acquired alloimmunity

Innate immunity represents the first line of defense against invading pathogens and noxious stimuli. The Toll-like receptors (TLRs) are essential innate immune receptors that alert the immune system to the presence of invading microbes. Emerging evidence shows that TLR signaling is important in allograft rejection. In a murine model, the rejection of minor mismatched allografts cannot occur in the absence of MyD88, an important TLR signal adaptor protein, owing to a defect in dendritic cell maturation, which leads to diminished T-helper cell type 1 immune responses. A recent clinical study also suggests that recipients with a mutant TLR4 genotype manifest reduced lung allograft rejection. Thus, innate immune signaling via TLRs is important for alloimmunity.