Dendritic cell depletion modifies corneal nerve activity and mice nociceptive behaviour

Purpose To determine the influence of resident dendritic cells (DC) in modulating corneal sensory nerve activity and its possible implications in basal tearing and pain. Methods C57BL/6 CD11c-DTR mice (both sexes, 5–6 months old) were used. Short-term (48 hr, STD) and long-term (8 days, LTD) DC depletion was done by bilateral subconjunctival injections of diphtheria toxin (DT) under anesthesia. Naïve mice (no DT-injections) served as control. Background and stimulus-evoked (cooling and heating ramps) activity of cold thermoreceptor nerve terminals was recorded ex vivo. Basal tearing rate was measured using commercial phenol red threads during 30 seconds in isoflurane-anesthetised animals. For monitoring the presence of spontaneous eye pain, the eye closure ratio (calculated by dividing the height by the width of the palpebral fissure measured in photographs) was used. Results STD produced a significant decrease in heating threshold (37.7 ± 0.7°C, n = 10 versus naïve, 41.6±1.8°C, n = 4 and LTD, 42.3±1°C, n = 8; p = 0.003, ANOVA). After LTD, lower temperature values were needed to reach both cooling threshold and peak frequency during cooling ramps (30.7 ± 0.5°C, n = 17 versus naïve, 32.3 ± 0.3°C, n = 20, and STD, 32.2±0.4°C, n = 17; p = 0.005, ANOVA). Basal tearing was slightly reduced in STD (2.8 ± 0.6 mm before DT versus 1.2 ± 0.38 mm at 48 hr, n = 6; p = 0.119, paired t-test) and signs of spontaneous pain were observed in both STD and LTD, being the eye closure ratio lower than before DT-injections (STD: 0.92 ± 0.01 before DT versus 0.18 ± 0.02 at 48 hr, n = 23, p ≤ 0.001, paired t-test; LTD: 0.9 ± 0.03 before DT versus 0.17 ± 0.02 at 48 hr versus 0.46 ± 0.07 at day 8, n = 3, p ≤ 0.001, RM One-way ANOVA). Conclusions Differences observed in nerve impulse activity and the subsequent changes in basal tearing and signs of spontaneous pain suggest a crucial role of DC in corneal nerve activity and ocular surface homeostasis. Supported by SAF2017-83674-C2-1-R and -2-R (AEI-ERDF), PROMETEO/2018/114 and ACIF/2017/169 (Generalitat Valenciana).