Changes in the intestinal lymphoid compartment throughout life: implications for the local generation of intestinal T cells

Impact of chronic and acute inflammation on extra- and intracellular iron homeostasis

Inflammation has a major impact on iron homeostasis. This review focuses on acute and chronic inflammation as it affects iron trafficking and, as a result, the availability of this essential micronutrient to the host. In situations of microbial infection, not only the host is affected but also the offending microorganisms, which, in general, not only require iron for their own growth but have evolved mechanisms to obtain it from the infected host. Key players in mammalian iron trafficking include several types of cells important to iron acquisition, homeostasis, and hematopoiesis (enterocytes, hepatocytes, macrophages, hematopoietic cells, and in the case of pregnancy, placental syncytiotrophoblast cells) and several forms of chaperone proteins, including, for nonheme iron, the transport protein transferrin and the intracellular iron-storage protein ferritin, and for heme iron, the chaperone proteins haptoglobin and hemopexin. Additional key players are the cell membrane-associated iron transporters, particularly ferroportin (FPN), the only protein known to modulate iron export from cells, and finally, the iron-regulatory hormone hepcidin, which, in addition to having antibacterial activity, regulates the functions of FPN. Interestingly, the impact of infection on iron homeostasis differs among pathogens whose mode of infection is mainly intracellular or extracellular. Understanding how inflammation affects each of these processes may be crucial for understanding how inflammation affects iron status, indicators of iron sufficiency, and iron supplementation during inflammation and how it may potentially result in a beneficial or detrimental impact on the host.

Age-dependent variation in the proportion and number of intestinal lymphocyte subsets, especially natural killer T cells, double-positive CD4+ CD8+ cells and B220+ T cells, in mice

The age-dependent variation in the proportion and number of lymphocyte subsets was examined at various extrathymic sites, including the liver, small intestine, colon and appendix in mice. In comparison with young mice (4 weeks of age), the number of total lymphocytes yielded by all tested organs was greater in adult (9 weeks) and old (40 weeks) mice. The major lymphocyte subset that expanded with age was interleukin-2 receptor (IL-2R) beta+ CD3int cells (50% of them expressed NK1.1) in the liver, whereas it was CD3+ IL-2Rbeta- NK1.1- cells at all intraepithelial sites in the intestine. Although NK1.1+ CD3+ cells were present at intraepithelial sites in the intestine, the proportion of this subset was rather low. The ratio of CD4 to CD8 tended to decrease among natural killer T (NKT) cells and T cells at all intraepithelial sites in the intestine with age. A unique population of double-positive CD4+ CD8+ cells in the small intestine increased in old mice. B220+ T cells were found mainly in the appendix and colon, and the proportion of these T cells decreased in old mice. Conventional NKT cells were very few in Jalpha281-/- and CD1d-/- mice in the liver, while NKT cells which existed in the appendix remained unchanged even in these mice. This was because unconventional CD8+ NKT cells were present in the intestine. The present results suggest that despite the fact that both the liver and intraepithelial sites in the intestine carry many extrathymic T cells, the distribution of lymphocyte subsets and their age-associated variation are site-specific.

Extrathymic TCR Gene Rearrangement in Human Small Intestine: Identification of New Splice Forms of Recombination Activating Gene-1 mRNA with Selective Tissue Expression

Two new 5'-untranslated region (5'UTR) exons were identified in the human gene for the lymphocyte-specific endonuclease recombination activating gene-1 (RAG1) required for the somatic recombination yielding functional Ag receptors. These 5'UTR exons were used in three different splice forms by jejunal lymphocytes of the T cell lineage. RAG1 mRNA containing the previously described 5'UTR exon was not expressed in these cells. Conversely, one of the new 5'UTR exons was not expressed in thymus. The new RAG1 mRNA splice forms were all expressed in immature T cells (CD2(+)CD7(+)CD3(-)). This cell population also expressed high levels of mRNA for the pre-T alpha-chain. In situ hybridization demonstrated jejunal cells expressing the new splice forms of RAG1 mRNA, both intraepithelially and in lamina propria. Pre-T alpha-chain mRNA-expressing cells were detected at the same sites. These results strongly suggest ongoing TCR gene rearrangement in human small intestinal mucosa, yielding T cells specially adapted for this environment. This seems to be achieved by two parallel processes, extrathymic T cell development and peripheral Ag-driven TCR editing.

An early expansion of CD8alphabeta T cells, but depletion of resident CD8alphaalpha T cells, occurs in the intestinal epithelium during primary simian immunodeficiency virus infection

OBJECTIVES

To evaluate changes in the phenotypic heterogeneity and function of CD8 T cells in the intestinal epithelium during primary SIV infection.

DESIGN

Previous studies have shown an increased prevalence of CD8 T cells in the intestinal epithelium in HIV and SIV infections. As intestinal CD8 T cells are a heterogeneous population we evaluated their phenotypic distribution (CD8alphabeta, CD8alphaalpha) and function [interferon (IFN)-gamma production] during primary SIV infection.

METHODS

The phenotype and functional potential of CD8 intestinal intraepithelial lymphocytes (IEL) prior to and following SIV infection were determined using flow cytometry.

RESULTS

IEL were found to harbor CD8alphabetaCD3, CD8alphaalphaCD3 and CD8alphaalpha+CD3- T-cell subsets. Most of the CD8CD4 double positive IEL expressed CD8alphaalpha homodimers. In primary SIV infection the frequency of CD8alphabetaCD3 T cells increased dramatically whereas the frequency of CD8alphaalpha T cells declined. A higher frequency of CD8alphabetaKi-67 IEL was observed following SIV infection suggesting that local cell proliferation might have contributed to an increased prevalence of CD8alphabeta IEL. In contrast, a severe depletion of CD8alphaalphaCD4 IEL occurred which contributed to the depletion of CD8alphaalpha IEL. The CD8alphabeta IEL were the major producers of IFN-gamma in the intestinal epithelium and the frequency of IFN-gamma-producing CD8alphabeta IEL was enhanced considerably in primary infection.

CONCLUSIONS

CD8alphabeta IEL may be important in generating early antiviral responses at the intestinal epithelium. However, alterations in CD8 T-cell subsets and their function may reflect early immunopathogenic events in the intestinal mucosa.