Background Secondary lymphoid organs are characterized by concordance in time, function and response to stimuli between the lymphatic and vascular system that allows tight regulation of cellular recruitment and exit from the immune station (1,2). Defective drainage of the inflammatory cells from sites of chronic inflammation has been suggested to play a role in the sustenance of the inflammatory process.
Objectives We used an inducible model of salivar gland ectopic lymphoneogenesis (3) in WT and KO mice, to evaluate whether similar events take place in the periphery and assess the modifications occurring to the vascular systems in different phases of the inflammatory process.
Methods Flow cytometry and histological analysis in WT and LTβRKO salivary glands, cannulated with 10^8 p.f.u. of adenovirus, was performed at different time points post cannulation (pc). Flow cytometry was used to identify in digested tissues the CD45-EPCAM-CD31+GP38- blood endothelial cells (BEC) and CD45-EPCAM-CD31+GP38+ lymphatic endothelia cells (LEC). Immunofluorescence (IF) for CD31 and Lyve was used to validate the FACS data and establish the histological relationship between the lymphatic and the vascular system within the inflamed samples.
Results By flow cytometry we observed a significant deflection in the percentage of blood endothelial cells in the early phases of the inflammatory process, followed by a progressive return to resting conditions during resolution. Conversely a significant increase in the number of lymphatic endothelial cells was observed in the early phase, followed by a significant decrease (p>0.01) at the peak of the inflammatory process and a drastic increase during resolution p= 0.01. Interestingly, while the first peak of lymphatic cells expansion corresponded to histological finding of a small number of lymphatic characterized by an enlarged lumen, the second peak was due to increase in the number of small lymphatics. This latest dramatic change in the number of the lymphatic vessels coincided with the infiltration of B-lymphocytes and peak of lymphoid cytokine lymphotoxin signal in the gland. Cannulation of LTβRKO, aimed to dissect the role of this molecule in lymphatic plasticity, demonstrated selective absence of the second peak of lymphatic proliferation and lack of lymphatic expansion by IF suggesting a key role for this molecule in lymphatic modification during resolution. Of note, LTβRKO mice did not show any significant difference as compared to the WT in the percentage of blood endothelial cells.
Conclusions This study shows that ectopic lymphoid follicles associated vasculogenesis fails to recapitulate the tight regulation observed in secondary lymphoid organs. Of interest, the observed changes in the lymphatic bed seems to be regulated by the presence of ectopically expressed lymphocyte derived lymphotoxin, suggesting a novel therapeutic role for this molecule in inflammation.
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Disclosure of Interest None Declared
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