Elsevier

Human Immunology

Volume 58, Issue 2, December 1997, Pages 83-90
Human Immunology

Human Small Intestinal Epithelial Cells Secrete Interleukin-7 and Differentially Express Two Different Interleukin-7 mRNA Transcripts: Implications for Extrathymic T-Cell Differentiation

https://doi.org/10.1016/S0198-8859(97)00230-9Get rights and content

Abstract

ABSTRACT: The small intestinal epithelium, composed of epithelial cells (EC) and intraepithelial T lymphocytes, is exposed to numerous ingested antigens. Small intestinal EC may act as accesory and/or antigen presenting cells for intestinal T cells, some of which may mature extrathymically and regulate local immunity and tolerance. Since interleukin-7 (IL-7) plays an essential role in T cell maturation and activation, we examined its expression by human small intestinal EC. IL-7 was detected by ELISA in supernatants from 4 of 4 epithelial layer (EpL) cultures. Using RT-PCR, IL-7 mRNA was detected in 4 EpL studied, and two distinct IL-7 transcripts were identified in 3 of the 4. The ratios of the intensities of the larger to the smaller bands varied amongst individuals. Furthermore, the intensity ratios were higher in whole-thickness intestine and lamina propria preparations than in their corresponding EpL. This is the first report of the expression of two IL-7 transcripts in human intestine and of IL-7 secretion by human small intestinal EpL cells. This supports the hypothesis that small intestinal EC may influence differentiation and/or activation of neighboring T cells. The differential expression of the two transcripts may have important implications for immune regulation in the intestinal epithelium.

Introduction

Interleukin-7 (IL-7) is a pleiotropic cytokine, initially described as a growth and differentiation factor for pre-B cells 1, 2and a proliferation factor for mature B cells [3], but has also been shown to have diverse effects on differentiation, growth and function of T cells 4, 5, 6, 7and cells of the myeloid lineage 8, 9. IL-7 supports growth and differentiation of thymocytes through many stages of maturation and is essential for early developmental processes such as the differentiation of pre-T cells into mature thymocytes. This latter function cannot be performed by any other known cytokine [7]. Although IL-7 can promote the differentiation of all T cell phenotypes, it appears to preferentially support the differentiation of pre-T cells into γδ T cell-receptor (TCR)+ 10, 11and CD8+ T cells 10, 12. Effects of IL-7 on mature T cells include upregulation of several activation markers 13, 14, 15, 16, induction of IL-2, IL-4, IL-6 and IFN-γ secretion 13, 17, proliferation 13, 14, 16, and cytotoxic function 18, 19, 20.

Recent evidence has indicated that the small intestine may be a site of extrathymic T cell differentiation. Phenotypic analyses of human intraepithelial lymphocyte (IEL) populations in the small intestine have revealed the presence of a subpopulation of T cells expressing the CD8αβcells and double negative CD4 CD8 T cells 21, 22. Murine studies have suggested that these intestinal T cell populations are of extrathymic origin 23, 24. The expression of the recombinase activation genes RAG1 and RAG2 mRNA in the human small intestinal epithelial layer [25]is a further indication that T cell maturation may be taking place locally.

T cell maturation and differentiation is dependent upon several components provided by other cells, including cytokines and surface molecules that drive the selection of a specific mature T cell repertoire. In the thymus, many of the components are provided by cortical and medullary EC [26]but it is not known if small intestinal EC have an influence on the distinct repertoire of T cells found in the intestinal mucosa. Normal small intestinal, but not colonic, EC express major histocompatibility complex class II molecules 27, 28, 29and CD1 [30]. These cells have been shown to produce IL-6 31, 32and express IL-1 mRNA [32]. Therefore, small intestinal EC are able to generate many of the elements produced by EC in the thymus that have been identified as important for T cell maturation.

An essential requirement for T cell differentiation in the thymus is local IL-7 production by subcapsular and cortical EC [26]; therefore, we have investigated whether human small intestinal EC are capable of producing IL-7 mRNA and protein. IL-7 production by human small intestinal EC would indicate that these cells may influence local T cell activation and that they have the potential to provide an environment suitable for local maturation of immature IEL.

Section snippets

Human Tissue Specimens

Five duodenal biopsies were obtained from each of two consenting patients (individuals 1 and 2) undergoing oesophagogastroduodenoscopy, in whom small intestinal disease was excluded by endoscopic and histological analysis. Surgical specimens of uninvolved ileal tissue were obtained from a further two patients (individuals 3 and 4) undergoing resection for colonic cancer and were dissected to fragments smaller than 1 cm2. Biopsies or surgical specimen fragments were collected in ion-free Hanks’

Detection of IL-7 mRNA

Two DNA fragments were obtained by PCR amplification of reverse transcribed mRNA extracted from bone marrow (Fig. 1) and thymus specimens using the IL-7 primers. These included the predicted 620 bp fragment of IL-7 cDNA (band 1) and a smaller DNA product (band 2), corresponding to a 488 bp fragment produced by alternative IL-7 mRNA splicing [36]. Both bands were expressed in each of the four bone marrow samples (individuals 5–8) and in the thymus (individual 9) used as positive controls for

Discussion

The results of the present study indicate that the human small intestinal epithelial layer produces IL-7. This has been demonstrated both at the level of gene expression and protein secretion. The levels of IL-7 secreted by normal small intestinal epithelial layer specimens are comparable to those secreted by bone marrow cells and PBMC. We also have shown differential expression of two distinct IL-7 transcripts in the epithelial layer. Nucleotide sequence analysis confirmed that the 620 bp PCR

Acknowledgements

We would like to thank Mr. Ken Mealy for providing us with the surgical specimens, Dr. David Noone for performing the automatic sequencing, R&D Systems for performing the IL-7 ’ELISA’ assays and the Irish Health Research Board for financial support.

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