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Characterisation of the normal conjunctival leukocyte population
M Hingorani1, D Metz, S L Lightman
1Institute of Ophthalmology, London, UK.
Insights
Normal conjunctival tissue contains scattered lymphoid cells, primarily T cells and macrophages. Leukocyte populations differ between bulbar and tarsal areas, with more cells in the bulbar region, suggesting increased antigen exposure.
Area of Science:
- Ocular immunology
- Immunohistochemistry
- Mucosal immunology
Background:
- Lymphoid cells are integral to mucosal tissues, forming organized structures like Mucosa-Associated Lymphoid Tissue (MALT).
- Understanding normal conjunctival lymphoid tissue is crucial for studying inflammatory eye conditions.
Purpose of the Study:
- To characterize the leukocyte populations in the normal human tarsal and bulbar conjunctiva.
- To establish a baseline for non-pathological conjunctival immune cells.
Main Methods:
- Immunohistochemistry was employed to analyze conjunctival tissue from healthy individuals.
- Leukocyte subsets, including T cells (CD3+, CD4+, CD8+), B cells, NK cells (CD57+), neutrophils, macrophages, and mast cells, were identified and quantified.
Main Results:
- CD3+ T cells and macrophages were the most abundant leukocytes, present in both epithelium and substantia propria.
- Leukocyte numbers, particularly CD3+ T cells and CD57+ NK cells, were significantly higher in the bulbar conjunctiva compared to the tarsal conjunctiva.
- CD8+ T cells predominated in the epithelium, while CD4+ T cells were more numerous in the substantia propria. CD45Ro+ T cells were prevalent in both compartments.
- Organized conjunctival lymphoid tissue (CALT) was rarely observed, suggesting it may not be universally present in humans.
Conclusions:
- The conjunctiva harbors a distinct leukocyte population with regional variations, potentially reflecting differential antigen exposure.
- The findings provide a foundational understanding of normal conjunctival immunity, essential for diagnosing and managing ocular surface diseases.
- The limited presence of organized CALT challenges the notion of its universal occurrence in humans.
Abstract:
Lymphoid cells are present in normal mucosal tissue as scattered cells or organised into MALT. Knowledge of the non-pathological conjunctival lymphoid tissue is a vital basis for the further study of ocular surface inflammatory disorders. Therefore, we used immunohistochemistry to examine the leukocyte population of tarsal and bulbar conjunctiva from normal patients with no ocular or systemic inflammatory disease. CD3+ T cells were the most frequently occurring, and macrophages the second most frequently occurring, conjunctival cell type and were seen in the epithelium and substantia propria. There were greater numbers of leukocytes in the bulbar than in the tarsal area and this reached statistical significance for CD3+ T cells and CD57+ NK cells. In both bulbar and tarsal conjunctiva, B cells and neutrophils were seen in the epithelium and substantia propria, but plasma cells, NK cells and mast cells were present only in the substantia propria. No eosinophils were seen. CD8+ cells outnumbered CD4+ cells in the epithelium (CD4:CD8 0.3) but this was reversed in the substantia propria (CD4:CD8.1.3 tarsal, 2.0 bulbar). Most epithelial T cells and half the stromal T cells were CD45Ro+. IL-2R (CD25) staining was infrequent in the tarsal but not the bulbar area. The greater number and activation of T cells in the bulbar region may relate to greater antigen exposure. HLA-DR+ cells were mainly macrophages, but also included dendritic cells in the epithelium and a few epithelial cells. The conjunctival lymphocytes do have certain features of MALT cells, apart from mucosal recirculation, homing and promotion of sIgA production. Conjunctival IEL, like gut IEL, are predominantly CD8+, are found in the basal epithelium, are HML-1+, and have very high expression of CD45Ro. Substantia propria lymphocytes have more equal CD4 and CD8 numbers and frequently express CD45Ro. We found only one example of organised conjunctival lymphoid tissue in our study and suggest that the presence of CALT in humans is not universal.