The role of the lymphatic system in inflammatory-erosive arthritis

Abstract

Rheumatoid arthritis (RA) is a prevalent inflammatory joint disease with enigmatic flares, which causes swelling, pain, and irreversible connective tissue damage. Recently, it has been demonstrated in murine models of RA that the popliteal lymph node (PLN) is a biomarker of arthritic flare, as it "expands" in size and contrast enhancement during a prolonged asymptomatic phase, prior to when it "collapses" with accelerated synovitis and joint erosion. This PLN collapse is associated with adjacent knee flare, decreases in PLN volume and contrast enhancement, lymphatic pulse and pumping pressure, and an increase in PLN pressure. Currently, it is known that PLN collapse is accompanied by a translocation of B cells from the follicles to the sinuses, effectively clogging the lymphatic sinuses of the PLN, and that B cell depletion therapy ameliorates arthritic flare by eliminating these B cells and restoring passive lymphatic flow from inflamed joints. Here we review the technological advances that have launched this area of research, describe future directions to help elucidate the potential mechanism of PLN collapse, and speculate on clinical translation towards new diagnostics and therapies for RA.

Keywords: Flare; Lymph node; Lymphatic vessel; Rheumatoid arthritis.

Figure 1. Routes of transport afferent and efferent to the PLN affect lymph node contrast enhancement (LNCE)

CE-MRI is preformed of the PLN by injecting gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) retro-orbitally and allowing it to circulate for five minutes before beginning the MRI scan. This timepoint was chosen because the CE signal of the tissue is stable for the duration of the scan. Following the scan, LNCE is calculated by measuring the intensity of the LN and normalizing it to that of the surrounding muscle. LNCE is likely dependent on the routes of transport to the PLN: sources include arterial blood vessels (BV) from the Gd-DTPA remaining in circulation from the retro-orbital injection, and afferent lymphatic vessels (LV) from the Gd-DTPA leaking from blood capillaries in the distal tissue and being taken up by afferent LV (A). Gd-DTPA arriving from afferent LV will primarily reside within the sinuses (yellow arrow, A), correlating to where the signal is seen in the CEMRI image (yellow arrow, B) which surrounds the dark medulla (M) that excluded the contrast agent. Drainage of Gd-DTPA from the PLN can occur from venous return of the PLN and efferent lymphatic vessels (A). Due to the short time scale in which CE-MRI is preformed, clearance of the Gd-DTPA from the PLN is not observed.

Figure 2. CXCL13 secreting macrophages are the prominent cells in TNF-Tg popliteal lymph node (PLN) sinuses

PLN were harvested from TNF-Tg mice and were processed for immunohistochemistry against F4/80 (A, brown) and CXCL13 (B, green). Representative images are shown at 20× and 40×, respectively. Note the presence of the F4/80⁺ macrophages within the sinus (A) and the sporadic staining of CXCL13 (B), likely due to cells within the sinuses and not within the T/B cell zone.

Figure 3. Overview of the changes that occur in expanding and collapsed TNF-Tg mice

The PLN drains the ankle synovium while the iliac lymph node (ILN) drains the knee synovium. During the expanding phase, both PLN and ILN are large in volume with open sinuses, and afferent lymphatic vessels exhibit contractions. This keeps the synovial volume stable and the small amount of synovitis at this point is due to overwhelming of the capacity of the lymphatic system. This mild disease transitions rapidly to advanced inflammatory-erosive arthritis following the simultaneous collapse of ipsilateral PLN and ILN via poorly understood mechanisms. At this point, there is a lack of lymphatic contraction, the PLN and ILN decrease in size and show B cell clogging of the sinuses. This lack of lymphatic transport results in a very large synovial volume and pannus formation in the knee and ankle, commonly known as arthritic flare.

Figure 4. A schematic of lymphatic pulsing under normal and inflammatory conditions

(A) Under normal condition, eNOS is induced by shear stress generated by lymphatic flow in endothelial cells (ES, orange) to maintain a regular lymphatic pulse via nitric oxide (NO, yellow arrows) regulation of lymphatic smooth muscle cell (SMC, green) contractions. (B) During chronic inflammatory conditions, such as inflammatory-erosive arthritis, high levels of NO produced by CD11b⁺/Gr-1⁺/iNOS⁺ cells (blue) that have attached to the lymphatic vessel squelching the eNOS NO gradient such that SMCs are in a constant state of relaxation, therefore causing the loss of lymphatic pulse and reduction of lymphatic clearance.