Lymphatic imaging to assess rheumatoid flare: mechanistic insights and biomarker potential

Abstract

Proliferation of draining lymphatic vessels coupled with dynamic changes in lymph node volume and flow are characteristic features in rheumatoid arthritis (RA). Furthermore, impaired lymph egress from inflamed synovium is associated with joint flare in murine models of inflammatory-erosive arthritis. Unfortunately, advances towards a greater understanding of lymphatic changes in RA pathogenesis have been slow due to the absence of outcome measures to quantify lymphatic function in vivo. While lymphoscintigraphy is the current standard to assess lymphedema and sentinel lymph nodes in cancer patients, its sensitivity and specificity are inadequate to study lymphatics in RA. The emergence of high-resolution MRI, power Doppler ultrasound, and near-infrared imaging that permits real-time quantification of lymphatic function in animal models has been a major advance, and these techniques have produced a new paradigm of altered lymphatic function that underlies both acute arthritic flare and chronic inflammation. In acute flare, lymphatic drainage increases several fold, whereas no lymphatic contractions are detected in lymph vessels draining chronic arthritic joints. Moreover, these outcomes are now being adapted to study lymphatics in RA towards the development of novel biomarkers of arthritic flare and the discovery of new therapeutic targets. In particular, interventions that directly increase lymphatic egress from diseased joints by opening collateral lymphatic vessels, and that restore lymphatic vessel contractions, provide novel therapeutic approaches with potential for minimal toxicity and immunosuppression. To summarize the origins of this field, recent advances, and future directions, we herein review: current knowledge of lymphatics in RA based on classic literature; new in-vivo imaging modalities that have elucidated how lymphatics modulate acute versus chronic joint inflammation in murine models; and how these preclinical outcome measures are being translated to study lymphatic function in RA inflammation and how effective RA therapies alter lymphatic flow and lymph nodes draining flaring joints.

Trial registration: ClinicalTrials.gov NCT02680067 . Registered 7 December 2015; ClinicalTrials.gov NCT01098201 . Registered 30 March 2010; and ClinicalTrials.gov NCT01083563 . Registered 8 March 2010.

Fig. 1

Increased lymphatic flow from flaring joints normalizes with effective anti-TNF therapy. Images of ⁹⁹Tc sulfur colloid tracing of the lower extremity to the pelvis obtained from a RA patient with asymmetric knee flare enrolled in a clinical study to assess the effects of anti-TNF therapy on arthritis and lymphatics (ClinicalTrials.gov NCT01083563). Note the bright ⁹⁹Tc sulfur colloid signal in the inguinal (arrows) and popliteal (arrowheads) areas on both lower extremities pre treatment, and the decreased signal post treatment, which resulted in normalization of the tt from 15 minutes to 60 minutes. Schematic diagram between the lymposcintigrams orients the regions of interest. R right leg, L left leg

Fig. 2

Expanding versus collapsed popliteal LNs in early versus late-stage RA knee flare. MRIs were obtained from RA patients with new-onset (a,b) and long-standing (>20 years) (c,d) asymmetric knee flare, who were enrolled in a phase 4 clinical trial to assess the effects of anti-TNF therapy on PLN volume (ClinicalTrials.gov NCT01098201). The 2D sagittal views (a,c) and 3D volume renderings (b,d) illustrate the dramatic differences between early RA flare with expanded PLN (pink, green and yellow arrows corresponding to pseudocolored PLN) adjacent to highly inflamed synovium and focal erosions in the lateral femoral condyle (*), compared with the barely visible collapsed PLN (blue and yellow arrows corresponding to pseudocolored PLN) adjacent to advanced pannus tissue and extensive bone erosions

Fig. 3

Clinical NIR imaging to quantify lymphatic flow in the upper extremity. A custom NIR imaging system (FD-1665; FluxData Inc., Rochester, NY, USA) was used to assess lymphatic contraction frequency in a healthy human subject after ICG injection in the second, third, and fourth web spaces of both hands as described in ClinicalTrials.gov NCT2680067. NIR excitation (<1.8 mW/cm²) was monitored with a Thorlabs PM16-121 power meter adjacent to the first web space. After injections, the upper extremities were imaged for 10 minutes to observe lymphatic flow. Visible and NIR (>800 nm) images were collected simultaneously; ICG fluorescence images were used to pseudocolor the visible image to provide anatomic localization (a). The ROI was positioned over lymphatics to calculate the mean pixel value for each NIR frame; peaks associated with lymphatic contractions were counted to calculate cpm. Representative images of the left hand (b) obtained from real-time video (Additional file 1: Movie 1) and right antecubital fossa (c) (Additional file 2: Movie 2) with the ROI and the respective cpm (green)