Tolerance to liver-specific antigens

Functional responses of resident human T cells in intact liver tissue

The liver contains a rich mix of T cells, including activated T cells, tissue-resident memory T cells and cells undergoing apoptosis. When antigens are presented in this milieu the default result is functional tolerance. T cell tolerance in the liver could be constitutive, or it could be adaptive, in which case liver cells would become unresponsive after encountering antigen in the liver context. To test this model, we evaluated the potential of human liver T cells to respond to T cell receptor ligation in liver tissue slice cultures. These T cells contained an actively motile subset of CD4+ T cells marked by CCR7 and CD62L, and fully functional subsets of CD4+ and CD8+ T cells that synthesized effector cytokines but subsequently assumed an exhausted phenotype. These data favor the model that human liver T cells are not constitutively tolerant but undergo adaptive tolerance after activation.

Cellular and molecular mechanisms of liver tolerance

The liver exhibits a distinctive form of immune privilege, termed liver tolerance, in which orthotopic liver transplantation results in systemic donor-specific T-cell tolerance, while antigens introduced either into hepatocytes or via the portal vein also cause tolerance. Here we argue that the fundamental mechanism driving liver tolerance is likely to be the continuous exposure of diverse liver cell types to endotoxin, derived from the intestinal bacteria. This exposure promotes the expression of a set of cytokines, antigen-presenting molecules, and costimulatory signals that impose T-cell inactivation, partly via effects on liver antigen-presenting cells. The evidence favors clonal deletion mechanisms and is consistent with a role for regulatory T cells but does not support either anergy or immune deviation as important factors in liver tolerance.

Expression of a Schistosoma mansoni 28-kilodalton glutathione S-transferase in the livers of transgenic mice and its effect on parasite infection

Schistosomiasis is a debilitating tropical disease for which an effective vaccine is needed. A 28-kDa glutathione S-transferase from Schistosoma mansoni (Sm28GST) has been shown to induce protective immunity. Sm28GST possesses significant sequence identity to mammalian GST isoforms. In order to study self-reactivity in mice immunized with Sm28GST and the concomitant phenomena of immune tolerance and epitope suppression, as well as their consequences for the protective immunity induced by this vaccination, we developed transgenic (Tg) mice that express Sm28GST under the control of a part of the mouse transferrin gene promoter. A study of (P28)Tg mice showed that the expression of Sm28GST was strictly localized in pericentrolobular hepatocytes. No histological change, inflammatory infiltrates, or modification of seric L-aspartate: 2-oxoglutarate aminotransferase concentration was observed over an 18-month period, despite a cross-reactivity between Sm28GST and a mouse molecule of 30 kDa. The immunoglobulin G anti-Sm28GST response of (P28)Tg mice immunized with recombinant Sm28GST was lower (P < 0.001) than that observed in non-(P28)Tg littermates and inversely proportional of Sm28GST liver expression. The response of non-(P28)Tg mouse spleen cells to Sm28GST stimulation was greater (P < 0.01) than that observed with (P28)Tg mouse spleen cells. (P28)Tg mice infected with 40 S. mansoni furcocercariae harbored more worms (P < 0.05) than did non-(P28)Tg control mice. The increase in the level of infection in (P28)Tg mice was reflected in concomitant increases in the numbers of adult worms and schistosome eggs found in livers and intestines after whole-body perfusion at 56 days postinfection, but no relative increase in the fertility of individual female worms was observed. The results obtained argue for the involvement of Sm28GST in reducing levels of infection and support the view that this enzyme has a central role in the maintenance of parasite viability, at least during its migration through host tissues.

Compartmentalization of the peripheral immune system

The periphery of the immune system--as opposed to the central lymphoid organs--contains inhomogeneously distributed B and T cells whose phenotype, repertoire, developmental origin, and function are highly divergent. Nonconventional lymphocytes bearing a phenotype that is rare in the blood, spleen, or lymph nodes of undiseased individuals are encountered at high frequency in different localizations, e.g., alpha/beta TCR+CD4-CD8- cells in the bone marrow and gut epithelium, particular invariant gamma/delta TCR+CD4-CD8 alpha+CD8 beta- and gamma/delta TCR+CD4-CD8 alpha-CD8 beta- T cells in various epithelia, or CD5+ B cells in the peritoneum. The antigen receptor repertoire is different in each localization. Thus, different gamma/delta TCR gene products dominant in each site, and the proportion of cells expressing transgenic and endogenous alpha/beta TCR and immunoglobulin gene products follows a gradient, with a maximum of endogenous gene expression in the peritoneum, intermediate values in other peripheral lymphoid organs (spleen, lymph nodes), and minimum values in thymus and bone marrow. Forbidden T cells that bear self-superantigen-reactive V beta gene products are physiologically detected among alpha/beta TCR+CD4-CD8- lymphocytes of the bone marrow, as well as in the gut. Violating previous ideas on self-tolerance preservation, self-peptide-specific gamma/delta T cells are present among intestinal intraepithelial lymphocytes, and CD5+ B cells produce low-affinity crossreactive autoantibodies in a physiological fashion. It appears that, in contrast to the bulk of T and B lymphocytes, certain gamma/delta and alpha/beta T cells found in the periphery, as well as most CD5+ B cells, do not depend on the thymus or bone marrow for their development, respectively, but arise from different, nonconventional lineages. In addition to divergent lineages that are targeted to different organs guided by a spatiotemporal sequence of tissue-specific homing receptors, local induction or selection processes may be important in the diversification of peripheral lymphocyte compartments. Selection may be exerted by local antigens, antigen-presenting cells whose function varies in each anatomical localization, cytokines, and cell-matrix interactions, thus leading to the expansion and maintenance of some clones, whereas others are diluted out or deleted. The spatial compartmentalization of lymphocytes in different microenvironments has major functional consequences and leads to a partial fragmentation of immunoregulatory circuits at the local level. Lymphocytes residing in certain antigen-exposed compartments are likely to combat tissue-specific pathogens or self-proteins.(ABSTRACT TRUNCATED AT 400 WORDS)