Glycosylphosphatidylinositol-Anchored Mucin-Like Glycoproteins from Trypanosoma cruzi Bind to CD1d but Do Not Elicit Dominant Innate or Adaptive Immune Responses Via the CD1d/NKT Cell Pathway

CD1d‐restricted T‐cell subsets and dendritic cell function in autoimmunity

CD1-restricted T cells have been shown to play a critical role in host defence, tumour surveillance, and maintenance of tolerance. However, immunologic outcomes resulting from activation of CD1d-restricted T cells can be either beneficial or deleterious. A major mechanism by which CD1d-restricted T cells are thought to exert immunoregulatory control is via effects on dendritic cell (DC) differentiation and migration. Important functional subsets of CD1d-restricted T cells are also known to exist and the potential implications for preferential subset activations are discussed.

The Third Way: Progress on pathways of antigen processing and presentation by CD1

CD1 proteins are a third family of antigen presenting molecules that bind bacterial and autologous lipid antigens for presentation to T cells. With the solution of the crystal structures of several complexes of CD1 molecules with lipids, a greater appreciation has been gained of the adaptability of CD1 in binding lipid antigens with diverse structural features. Biochemical studies of the interactions between the TCR and CD1-lipid complexes have revealed striking contrasts with TCR that bind to peptides presented by MHC-encoded class I and class II molecules. The sphingolipid activating proteins (SAP) have recently been found to facilitate the transfer of lipid antigens onto CD1 molecules. This helps to provide an explanation as to how the thermodynamic barrier, caused by loading hydrophobic lipid antigens in a hydrophilic environment, can be overcome. Mechanisms of CD1 endosomal trafficking are being delineated, including the means by which adaptor proteins induce the localization of some types of CD1 molecules to lysosomes, where they bind antigens. Unlike MHC class I and class II proteins, specialized molecules that function solely in chaperoning CD1 molecules, or in facilitating their antigen loading, have not been found. This suggests that the CD1 antigen presenting system, which diverged early in vertebrate evolution from MHC antigen presenting molecules, is a simpler system with a character closer to the primordial antigen presenting function.

Regulation of immunity and pathogenesis in infectious diseases by CD1d-restricted NKT cells

CD1d-restricted NKT cells are emerging as an unusual lymphoid lineage with important immunoregulatory properties. To date, much of our understanding of the biology of the CD1/NKT system comes from studies that utilise non-natural glycolipid ligands. Recent evidence suggests that NKT cells play an important role in the response to pathogens, manifesting a range of functions including cytotoxicity, help for antibody formation and regulation of Th1/Th2 differentiation. Infectious disease models provide appropriate physiological and pathophysiological systems to explore the biological roles of this lineage in immunity and disease. Novel insights are emerging from infection models, particularly with respect to the nature of ligands recognised by the T cell receptor of NKT cells, and to the role of diverse non-T cell receptor NK activation and inhibitory receptors in regulation of the lineage. Such insights have the potential to add considerably to our understanding of the CD1/NKT cell system and to the immunology and pathogenesis of infectious diseases.

Impaired Production of Proinflammatory Cytokines and Host Resistance to Acute Infection withTrypanosoma cruziin Mice Lacking Functional Myeloid Differentiation Factor 88

Studies performed in vitro suggest that activation of Toll-like receptors (TLRs) by parasite-derived molecules may initiate inflammatory responses and host innate defense mechanisms against Trypanosoma cruzi. Here, we evaluated the impact of TLR2 and myeloid differentiation factor 88 (MyD88) deficiencies in host resistance to infection with T. cruzi. Our results show that macrophages derived from TLR2 (-/-) and MyD88(-/-) mice are less responsive to GPI-mucin derived from T. cruzi trypomastigotes and parasites. In contrast, the same cells from TLR2(-/-) still produce TNF-alpha, IL-12, and reactive nitrogen intermediates (RNI) upon exposure to live T. cruzi trypomastigotes. Consistently, we show that TLR2(-/-) mice mount a robust proinflammatory cytokine response as well as RNI production during the acute phase of infection with T. cruzi parasites. Further, deletion of the functional TLR2 gene had no major impact on parasitemia nor on mortality. In contrast, the MyD88(-/-) mice had a diminished cytokine response and RNI production upon acute infection with T. cruzi. More importantly, we show that MyD88(-/-) mice are more susceptible to infection with T. cruzi as indicated by the higher parasitemia and accelerated mortality, as compared with the wild-type mice. Together, our results indicate that T. cruzi parasites elicit an alternative inflammatory pathway independent of TLR2. This pathway is partially dependent on MyD88 and necessary for mounting optimal inflammatory and RNI responses that control T. cruzi replication during the early stages of infection.

Sorting out self and microbial lipid antigens for CD1

CD1 proteins mediate T cell activation in response to self and foreign lipids, including lipid antigens from the intracellular pathogen Mycobacterium tuberculosis. During natural infections, myeloid cells migrate to sites of infection and use microbial pattern recognition receptors to internalize live bacteria and lipid antigens into the endosomal network. New studies show that certain CD1 proteins are particularly receptive to binding lipid antigens in the low pH environment of endosomes. Therefore, the endosomal network may represent a depot for concentrating and then selectively presenting exogenous foreign lipid antigens to T cells.

Understanding the function of CD1-restricted T cells

CD1 molecules bind foreign lipid antigens as they survey the endosomal compartments of infected antigen-presenting cells. Unlike T cells that recognize CD1-restricted foreign lipids, CD1-restricted T cells that are self-antigen-reactive function as 'auto-effectors' that are rapidly stimulated to carry out helper and effector functions upon interaction with CD1-expressing antigen-presenting cells. The functional distinctions between subsets of CD1-restricted T cells, and the pathways by which these cells both influence the inflammatory and tolerogenic effects of dendritic cells and activate natural killer cells and other lymphocytes, provide insight into how CD1-restricted T cells regulate antimicrobial responses, antitumor immunity and the balance between tolerance and autoimmunity.

Functional convergence of invertebrate and vertebrate cytokine-like molecules based on a similar lectin-like activity

It is generally accepted that the action of cytokines results from their binding to specific receptors. However, many cytokines possess lectin-like activity that may be essential for the expression of their full biological activities. This review focuses on the physiological relevance of the lectin-like activity of cytokines during the innate immune response in mammals, using TNF as an illustrative example. Moreover, we will show that TNF displays functional analogies with a defense molecule from the earthworm Eisenia foetida termed CCF. These analogies are not reflected by primary sequence homology between CCF and TNF but are particularly based on a similar lectin-like activity/domain. Hence, from a phylogenetic point of view, the lectin-like activity/domain of CCF and TNF may represent an essential recognition mechanism that has been functionally conserved during the innate immune response of invertebrates and vertebrates as a result of convergent evolution.