The NKT cells represent an intriguing subset of T lymphocytes coexpressing NKR-P1B/C (NK1.1)(CD161) that are type II membrane glycoproteins of the C-type lectin superfamily. The majority of NKT cells express an invariant (i) T cell receptor (TCR), typically comprising Vα14/Jα18 combined with a highly skewed TCRVβtowards Vβ8.2 in the mouse, and homologous chainVα24/Jα18 paired with Vβ11 in the human (iNKT cells). Unlike conventional T cells which are selected by polymorphic major histocompatibility complex (MHC) class I or class II expressed on the surface of thymic stromal cells, iNKT cells develop dependently on nonpolymorphic MHC class I-like antigen presentation molecule CD1d, which is expressed on the surface of CD4+8+ cortical thymocytes. In contrast to conventional T cells, which recognize peptide antigens presented by MHC class I or class II, iNKT cells recognize glycolipid antigens such as α-galactosylceramide (α-GalCer) which is a synthetic glycolipid originally isolated from a marine sponge, glycosphingolipids from Sphingomonas sp. and from Ehrlichia sp., diacylglycerol from Borrelia sp., phosphatidylinositol mannoside derived from Mycobacterium sp., and also endogenous lysosomal glycoshingolipid, isoglobotrihexosylceramide (iGb3) in the context of CD1d. Upon activation (e.g. TCR ligation), iNKT cells promptly secrete large quantities of the type 1 and type 2 cytokines, interferon(IFN)-γand interleukin(IL)-4, respectively and express cytolytic activity.
Thymus-independent T cells
The liver is a candidate site for extrathymic T cell development.
Both TCRα/β and TCRγ/δ cells have been identified in the liver of congenitally athymic nu/nu and thymectomized mice.
The number of these cells increases as a function of age although their appearance differs markedly between individual animals.
TCRα/β cells in the liver include autoreactive clones, probably due to escape from negative selection. Recombination-activating gene 1 expression has been found in the adult liver, suggesting that the liver serves as an alternative organ for T cell development. Thymus -independent liver T cells that encompass both CD4- 8- and CD8+ cells have been described.
Bacterial sepsis is a major health threat affecting 1 million patients in the United States and Europe every year. The resulting septic shock is mainly caused by an exaggerated systemic cytokine response to Gram-negative bacteria and their characteristic cell wall component, lipopolysaccharide (LPS). Experimental animal models have contributed profoundly to our understanding of the underlying molecular mechanisms. In mice, challenge with high doses of LPS results in a syndrome resembling septic shock. This model revealed that proinflammatory cytokines, in particular tumor necrosis factor(TNF)-α from macrophages and IFN-γ from NK1.1+ cells, played a key role in the disease process. The uncontrolled production of proinflammatory cytokines causes several pathophysiological reactions, including fever, hypotension and multiple organ failure, all of which ultimately form the septic shock syndrome, often with fatal outcome.
Because TNF-α and IFN-γ are central to lethality and organ dysfunction in experimental endotoxic shock, neutralization of these cytokines decreases mortality. The critical role of these cytokines in the pathogenesis of LPS-induced shock was confirmed using mice deficient for eitherof these cytokine receptors.
Intestinal intraepithelial lymphocytes
In the mouse, intestinal intraepithelial lymphocytes (i-IEL) comprise unique T cell populations which are distinct from mainstream T cells distributed in central lymphoid organs. The i-IEL encompass roughly equal numbers of TCRα/β cells and TCRγ/δ cells, and most of them coexpress CD8α.
CD8α+ TCRα/β i-IEL segregate into two populations on the basis of CD8β expression; one expressing CD8β (CD8α/β heterodimer) and the other not (CD8α/α homodimer). CD8α/β+ TCRα/β i-IEL are considered identical with mainstream T cells in central lymphoid organs which develop in a thymus-dependent manner, although some CD8α/β+ TCRα/β i-IEL have been suggested to develop in a thymus-independent fashion. In contrast, CD8α/α+ TCRα/β cells are prominent in the gut and are considered to develop in intestinal epithelial layers independently from a thymus. In contrast to CD8α+ TCRα/β i-IEL, virtually no CD8α+ TCRγ/δ i-IEL express CD8β and the development of this subset is considered thymus independent.
CD8+ T cells
Most CD8+ T cells in central lymphoid organs express a heterodimer comprising CD8α and CD8β. The development of CD8α/β+ T cells is controlled by β2-microglobulin (β2m)-associated classical MHC class I and is thymus dependent. A second population expresses CD8α only (probably CD8α/α homodimer) and is considered thymus independent.β2m gene disruption mutant (β2m-/-) mice lack surface expression of classical MHC class I and are consequently devoid of conventional CD8α/β+ T cells. Yet in these mice, a distinct CD8+ T cell population has been identified which expresses CD8α, but not CD8β. The CD8α+ β- T cells express either TCRα/β or TCRγ/δ.
Leukocyte function-associated antigen (LFA)-1
Leukocyte function-associated antigen (LFA)-1 (CD11a/CD18) is a leukocyte-specific cell adhesion molecule which belongs to the β2 subfamily of integrin adhesion receptors and is considered to play a pivotal role in mediating the firm adhesion of leukocytes to endothelial cells, leading to the subsequent leukocyte transmigration. LFA-1 is expressed on the surface of various cell populations such as T cells including NKT cells, B cells, macrophages, granulocytes, dendritic cells, and NK cells. In the liver, LFA-1 is also expressed on sinusoidal lining cells such as Kupffer cells. In the mouse, intercellular adhesion molecule (ICAM)-1 (CD54) and ICAM-2 (CD102) differentially expressed on leukocytes, epithelial cells, endothelial cells, and fibroblasts are a ligand for LFA-1, and LFA-1/ICAM interactions play a crucial role in leukocyte recirculation and migration.
T helper cells
Th cells segregate into two populations on the basis of their cytokine profile.
Th1 cells produce IL-2 and IFN-γ and play a key role in protection against intracellular bacteria, whereas Th2 cells produce IL-4 and IL-5 which are essential for antibody secretion and for protection against helminths.
The induction of a Th1-dominated immune response is promoted by endogenous IL-12, a heterodimeric cytokine produced by macrophages and B cells, which augments IFN-γ secretion by NK cells and T cells.
In contrast, Th2 cell differentiation is favored by IL-4 produced by conventional CD4+ T cells, Fc εreceptor cells such as mast cells, basophils and eosinophils and also NKT cells. Thus, Th cell polarization is determined at the outset of infection by the presence of particular cytokines.
L. monocytogenes is a Gram-positive facultative intracellular bacterium, which preferentially propagates in macrophages and liver parenchymal cells. Cells of the innate immune system play a pivotal role as a first line of defense against L. monocytogenes and among these, granulocytes and macrophages play a central role. The primary control of L. monocytogenes infection depends on the ability of the host to induce efficient type 1-immune responses, and interferon (IFN)-γ secreted from natural killer (NK) cells and Th 1 cells plays a critical role in protection. In contrast, Th2 cells secreting IL-4 exacerbate disease. IL-12 secreted by macrophages, dendritic cells and granulocytes is also important for protection against L. monocytogenes infection, with the primary function of inducing IFN-γ secretion from responding cells including NK1.1+ cells and hence, participates in early resistance to L. monocytogenes infection.