Lymphedema affects up to 1 in 6 patients who undergo treatment for a solid tumor in the United States. Its prevalence has increased as more effective oncologic therapies have improved patient survival, but there remains no definitive cure. Recent research has elucidated new details in the pathogenesis of the disease and has demonstrated that it is fundamentally an immunologic process that ultimately results in inflammation, fibroadipose deposition, impaired lymphangiogenesis, and dysfunctional lymphatic pumping. These findings have allowed for the development of novel medical and surgical therapies that may potentially alter the standard of care for a disease that has largely been treated by compression. This review seeks to provide an overview of the emerging therapies and how they can be utilized for effective management of lymphedema.

Lymphedema results from lymphatic insufficiency leading to a progressive inflammatory process that ultimately manifests as discomfort, recurrent infections, and, at times, secondary malignancy. Collectively, these morbidities contribute to an overall poor quality of life. Although there have been recent advances in microsurgical interventions, a conservative palliative approach remains the mainstay of treatment for this disabling disease. The absence of a cure is due to an incomplete understanding of the pathophysiological changes that result in lymphedema. A histological hallmark of lymphedema is inflammatory cell infiltration and recent studies with animal models and clinical biopsy specimens have suggested that this response plays a key role in the pathology of the disease. The purpose of this report is to provide an overview of the ongoing research in and the current understanding of the inflammatory manifestations of lymphedema.

Epilepsy is a common neurological disorder with many causes. For temporal lobe epilepsy, antecedent insults are typically found. These risk factors include trauma or history of long fever-associated seizures (febrile status epilepticus) in childhood. Whereas the mechanisms by which such insults promote temporal lobe epilepsy are unknown, an extensive body of work has implicated inflammation and inflammatory mediators in both human and animal models of the disorder. However, direct evidence for an epileptogenic role for inflammation is lacking. Here we capitalized on a model where only a subgroup of insult-experiencing rodents develops epilepsy. We reasoned that if inflammation was important for generating epilepsy, then early inflammation should be more prominent in individuals destined to become epileptic compared with those that will not become epileptic. In addition, the molecular and temporal profile of inflammatory mediators would provide insights into which inflammatory pathways might be involved in the disease process. We examined inflammatory profiles in hippocampus and amygdala of individual rats and correlated them with a concurrent noninvasive, amygdalar magnetic resonance imaging epilepsy-predictive marker. We found significant individual variability in the expression of several important inflammatory mediators, but not in others. Of interest, a higher expression of a subset of hippocampal and amygdalar inflammatory markers within the first few hours following an insult correlated with the epilepsy-predictive signal. These findings suggest that some components of the inflammatory gene network might contribute to the process by which insults promote the development of temporal lobe epilepsy.

T cell-mediated responses have been implicated in the development of fibrosis, impaired lymphangiogenesis, and lymphatic dysfunction in secondary lymphedema. Here we show that CD4+ T cells are necessary for lymphedema pathogenesis by utilizing adoptive transfer techniques in CD4 knockout mice that have undergone tail skin and lymphatic excision or popliteal lymph node dissection. We also demonstrate that T cell activation following lymphatic injury occurs in regional skin-draining lymph nodes after interaction with antigen-presenting cells such as dendritic cells. CD4+ T cell activation is associated with differentiation into a mixed T helper type 1 and 2 phenotype, as well as upregulation of adhesion molecules and chemokines that promote migration to the skin. Most importantly, we find that blocking T cell release from lymph nodes using a sphingosine-1-phosphate receptor modulator prevents lymphedema, suggesting that this approach may have clinical utility.

Nonhematopoietic stromal cells in lymph nodes such as fibroblastic reticular cells (FRCs) can support the survival of plasmablasts and plasma cells [together, antibody-forming cells (AFCs)]. However, a regulatory function for the stromal compartment in AFC accumulation has not been appreciated. Here, we show that chemokine ligand 2 (CCL2)–expressing stromal cells limit AFC survival. FRCs express high levels of CCL2 in vessel-rich areas of the T cell zone and the medulla, where AFCs are located. FRC CCL2 is up-regulated during AFC accumulation, and we use lymph node transplantation to show that CCL2 deficiency in BP3+ FRCs and lymphatic endothelial cells increases AFC survival without affecting B or germinal center cell numbers. Monocytes are key expressers of the CCL2 receptor CCR2, as monocyte depletion and transfer late in AFC responses increases and decreases AFC accumulation, respectively. Monocytes express reactive oxygen species (ROS) in an NADPH oxidase 2 (NOX2)–dependent manner, and NOX2-deficient monocytes fail to reduce AFC numbers. Stromal CCL2 modulates both monocyte accumulation and ROS production, and is regulated, in part, by manipulations that modulate vascular permeability. Together, our results reveal that the lymph node stromal compartment, by influencing monocyte accumulation and functional phenotype, has a regulatory role in AFC survival. Our results further suggest a role for inflammation-induced vascular activity in tuning the lymph node microenvironment. The understanding of stromal-mediated AFC regulation in vessel-rich environments could potentially be harnessed to control antibody-mediated autoimmunity.

Patients who suffer from lymphedema have impaired immunity and, as a result, are at an increased risk for infections. Furthermore, previous studies have shown that lymphadenectomy impairs acquisition of adaptive immune responses and antibody production in response to foreign antigens. Although it is clear that antigen presentation in lymph nodes plays a key role in adaptive immunity, the cellular mechanisms that regulate impaired immune responses in patients with lymphedema or following lymphatic injury remain unknown. We have previously found that axillary lymph node dissection, both clinically and in a mouse model, results in a marked increase in the number of regulatory T cells in the ipsilateral limb. In this study, we focus on the role of regulatory T cells in immunosuppression and show that regulatory T-cell proliferation in tissues distal to site of lymphatic injury contributes to impaired innate and adaptive immune responses. More importantly, using Foxp3-DTR transgenic mice, we show that depletion of regulatory T cells in the setting of lymphatic injury restores these critical immune-mediated responses. These findings provide additional evidence that immune responses following lymphatic injury play a key role in mediating the pathology of lymphedema.

Proliferation of aberrant, dysfunctional lymphatic vessels around solid tumors is a common histologic finding. Studies have shown that abnormalities in lymphatic function result in accumulation of inflammatory cells with an immunosuppressive profile. We tested the hypothesis that dysfunctional lymphatic vessels surrounding solid tumor regulate changes in the tumor microenvironment and tumor-specific immune responses. Using subcutaneously implanted mouse melanoma and breast cancer tumors in a lymphatic endothelial cell-specific diphtheria toxin receptor transgenic mouse, we found that local ablation of lymphatic vessels increased peritumoral edema, as compared with controls. Comparative analysis of the peritumoral fluid demonstrated increases in the number of macrophages, CD4 + inflammatory cells, F4/80 + /Gr-1 + (myeloid derived suppressor cells), CD4 + /Foxp3 + (T regs) immunosuppressive cells and expression of inflammatory cytokines such as TNFα, IFNγ and IL1β following lymphatic ablation. Tumors grown in lymphatic ablated mice exhibited reduced intratumoral accumulation of cytotoxic T cells and increased tumor PD-L1 expression, causing rapid tumor growth, compared with tumors grown in nonlymphatic-ablated mice. Our study suggests that lymphatic dysfunction plays a role in regulating tumor microenvironments and may be therapeutically targeted in combination with immunotherapy to prevent tumor growth and progression.

T cells infiltrating lymphedematous tissues have a mixed T helper 1 (Th1) and Th2 differentiation profile. Treatment with neutralizing antibodies targeting cytokines that promote Th2 differentiation [interleukin 4 (IL-4) and IL-13] decreases the severity of lymphedema in preclinical models, suggesting that Th2 cells play a key role in the pathology of this disease. However, these previous studies do not address the contribution of Th1 cells and it remains unknown if IL-4 and IL-3 blockade acts primarily on T cells or decreases the pathological changes of lymphedema by other mechanisms. Therefore, this study sought to analyze the effect of lymphatic injury in transgenic mice with mutations that cause defects in Th1 and Th2 cell generation (T-bet knockout or T-betKO and STAT6 knockout or STAT6KO mice, respectively). Using both the mouse tail and popliteal lymph node dissection models of lymphedema, we show that Th2-deficient (STAT6KO) mice are protected from developing lymphedema, have decreased fibrosis, increased collateral vessel formation, and preserved collecting lymphatic vessel pumping function. In contrast, mice with defective Th1 cell generation (T-betKO) develop disease with the same severity as wild-type controls. Taken together, our results suggest that Th2 differentiation is necessary for development of lymphedema following lymphatic injury and that Th1 differentiation does not significantly contribute to the pathology of the disease. Such findings are important as immunotherapy directed at Th2 cells has been found to be effective in well-studied Th2-mediated diseases such as asthma and atopic dermatitis and may therefore be similarly useful for lymphedema management. Brief CommentaryBackground-Lymphedema management is limited due to a poor understanding of its pathophysiology. In this study, we seek to delineate the importance of T helper 1 (Th1) and Th2 cells in this disease by evaluating the effect of lymphatic injury on mice that have impaired Th1 or Th2 cell generation. We found that Th1 differentiation does not significantly contribute to the disease, whereas the inability to develop Th2 cells is protective.