Background : Proper function of lymphatic vessels is needed to limit the magnitude and duration of tissue inflammation, and in direct regulation of immune cell activity. Inflammatory states such as rheumatoid arthritis and psoriasis are associated with lymphatic dysfunction, but lymphatics in lupus models have not been well characterized. SLE patients are photosensitive, developing inflammatory skin lesions upon exposure to even ambient ultraviolet radiation (UVR). We hypothesized that lymphatic dysfunction may contribute to photosensitivity in lupus.
Methods : MRL/MpJ-Faslpr/lpr (lpr) mice and age-/sex-matched MRL controls were evaluated at baseline, 1-day, 1-week and 1-month after exposure to 2000-2500 J/m2 of UVB radiation. Lymphatic function was assessed with an intradermal injection of 1µL of 2% evans blue (EB) to the ear, followed by measurement of EB concentration in the draining lymph node (dLN) 1 minute later. Flow cytometry of ears and dLN allowed quantification of resident cell populations. Kruskal-Wallis test, followed by Dunn’s test for multiple comparisons were used to compare the groups; data is presented as mean ± SE, with a two-tailed p-value of <0.05 considered significant.
Results : At baseline, the effective flow of EB, calculated as EB concentration in the dLN, per ear lymphatic endothelial cell (LEC) number, is comparable between the 2 groups. Following UVR, in both lpr and control mice, there is an increase in the effective flow of EB to the dLN, that starts as early as 1 day, and continues to increase at 1 week, in similar levels between the 2 groups. At 1 month post-UVR, however, the flow continues to improve in the MRL control mice, but there is no similar improvement in the lpr mice. In fact, while at baseline, 1-day and 1-week the mean relative lymphatic flow in the lpr mice ranges from 1.6-, 2.5- and 0.9- fold of controls, respectively, at 1 month the effective flow in the lpr mice is only 5% that of controls (p=0.007).
Conclusions : Dermal lymphatic network of lupus models have never been characterized, despite evidence for a major role of lymphatics in the regulation of chronic inflammation and autoimmunity. Here we provide indications to impaired long-term response of lymphatic drainage in the lpr lupus strain, which can lead to reduced lymphatic regulation of the immune response, contributing to the unchecked skin inflammation known to occur in SLE.