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The Relative Contribution of Mast Cell Subsets to Conjunctival T_H2-like Cytokines

¹ From the Southampton Eye Unit; and the ² Departments of Pathology and ³ Immunopharmacology, Southampton University, Hampshire, United Kingdom.

PURPOSE. To investigate the distribution of the T-helper (T_H)2-like cytokines, interleukin (IL)-4, IL-5, IL-6, and IL-13 between mast cell subsets in conjunctival biopsy specimens from normal subjects and those with seasonal allergic conjunctivitis (SAC) during and outside of the grass pollen season.

METHODS. Sequential and double in situ hybridization (ISH) and immunohistochemistry (IHC) were performed on thin sections of human conjunctiva to determine the colocalization of the immunoreactivity of IL-4, IL-5, IL-6, and IL-13 to mast cell subsets in normal subjects and subjects with atopy and to detect IL-4 mRNA in conjunctival mast cells.

RESULTS. More than 90% of IL-4⁺–immunoreactive cells were observed to be mast cells in conjunctival biopsy specimens from all patient groups. The majority of IL-5⁺, IL-6⁺, and IL-13⁺ cells were also noted to be mast cells for each group. IL-4 preferentially colocalized to the tryptase⁺-chymase⁺ mast cell phenotype (MC_TC) with MC_TC cells comprising 93.3% of cytokine⁺ mast cells in symptomatic SAC (P = 0.0017), 89.2% in asymptomatic SAC (P = 0.0008), and 77.8% in normal subjects (P = 0.0472). IL-13 appeared to colocalize preferentially to the MC_TC phenotype and IL-5 and IL-6 to the MC_T phenotype. ISH showed that 75.8% of mast cells in normal subjects, 78.7% in subjects with symptomatic SAC, and 18.7% in subjects with asymptomatic SAC expressed mRNA for IL-4.

CONCLUSIONS. Conjunctival mast cells are an important source of IL-4, IL-5, IL-6, and IL-13 immunoreactivity, with preferential colocalization of IL-4 and IL-13 on the MC_TC subset and IL-5 and IL-6 to the MC_T subset. This evidence suggests that differences in protease phenotype may also reflect functional differences evidenced by the different patterns of cytokine distribution.