Background/Purpose: Rheumatoid arthritis (RA) is a chronic disease with episodic flares in affected joints.  These flares are associated with decreased lymphatic drainage.  In TNF-Tg mice, a model of inflammatory-erosive arthritis, the popliteal lymph node (PLN) expands in volume during the pre-arthritic “expanding” phase and then “collapses” during flare. Currently, the factors that cause PLN collapse are unknown.  Local changes in physiology are reasonable explanations given the finding that PLN and iliac lymph node collapse occurs in series along the ipsilateral axis in the setting of unaltered chronic inflammation and systemic autoimmunity.  Thus, we tested the hypothesis that PLN collapse and arthritic flare are associated with increased pressure and decreased blood flow within the efferent lymph node (PLN) of inflamed joints.

Methods: The PLNs of TNF-Tg mice (C57B6) were phenotyped as expanding or collapse with contrast-enhanced MRI.  Pressure measurements were performed by inserting a glass micropipette connected to a pressure transducer into the PLN of anesthetized WT and TNF-Tg mice. Blood flow within the lymph node and lymph node volume were measured by 3D power Doppler ultrasound and volumetric rendering in Amira.

Results:    Lymph node pressure was significantly decreased in expanding PLN of TNF-Tg mice vs. that of WT mice (3.4 ± 0.4 vs 6.7 ± 0.6 cmH₂O; p<0.01). The pressure in collapsed PLN of TNF-Tg mice was significantly greater vs. expanding node in the same mice (8.6 ± 1.7 vs. 3.4 ± 0.4 cmH₂O; p<0.05), but similar to WT node.  We also found that the volume of blood vessels positive for Doppler flow was significantly 6-fold less in collapsed vs. expanding PLN of TNF-Tg mice (0.050 ± 0.036 mm³ vs. 0.3042 ± 0.08192 mm³; p<0.05) (Figure).

Conclusion: Here we describe a novel approach to quantify lymph node pressure and blood flow within PLNs of WT and TNF-Tg mice. Our finding of significantly decreased lymphatic pressure during the pre-arthritic stage is consistent with the theory that lymphangiogenesis is a compensatory mechanism to prevent synovitis and joint damage during RA pathogenesis. Furthermore, increased pressure and decreased vascular flow observed in collapsed PLN is also consistent with this theory, and suggest that lymphatic pressure and blood flow within the efferent lymph nodes are potential biomarkers to assess RA progression and response to therapy, with the later achieved using clinically relevant non-invasive ultrasound.