J.T. O’Malley

Survival of tissue-resident memory T cells requires exogenous lipid uptake and metabolism

Tissue-resident memory T (TRM) cells persist indefinitely in epithelial barrier tissues and protect the host against pathogens. However, the biological pathways that enable the long-term survival of TRM cells are obscure. Here we show that mouse CD8+ TRM cells generated by viral infection of the skin differentially express high levels of several molecules that mediate lipid uptake and intracellular transport, including fatty-acid-binding proteins 4 and 5 (FABP4 and FABP5). We further show that T-cell-specific deficiency of Fabp4 and Fabp5 (Fabp4/Fabp5) impairs exogenous free fatty acid (FFA) uptake by CD8+ TRM cells and greatly reduces their long-term survival in vivo, while having no effect on the survival of central memory T (TCM) cells in lymph nodes. In vitro, CD8+ TRM cells, but not CD8+ TCM cells, demonstrated increased mitochondrial oxidative metabolism in the presence of exogenous FFAs; this increase was not seen in Fabp4/Fabp5 double-knockout CD8+ TRM cells. The persistence of CD8+ TRM cells in the skin was strongly diminished by inhibition of mitochondrial FFA β-oxidation in vivo. Moreover, skin CD8+ TRM cells that lacked Fabp4/Fabp5 were less effective at protecting mice from cutaneous viral infection, and lung Fabp4/Fabp5 double-knockout CD8+ TRM cells generated by skin vaccinia virus (VACV) infection were less effective at protecting mice from a lethal pulmonary challenge with VACV. Consistent with the mouse data, increased FABP4 and FABP5 expression and enhanced extracellular FFA uptake were also demonstrated in human CD8+ TRM cells in normal and psoriatic skin. These results suggest that FABP4 and FABP5 have a critical role in the maintenance, longevity and function of CD8+ TRM cells, and suggest that CD8+ TRM cells use exogenous FFAs and their oxidative metabolism to persist in tissue and to mediate protective immunity.

Clinically resolved psoriatic lesions contain psoriasis-specific IL-17–producing αβ T cell clones

In psoriasis, an IL-17–mediated inflammatory skin disease, skin lesions resolve with therapy, but often recur in the same locations when therapy is discontinued. We propose that residual T cell populations in resolved psoriatic lesions represent the pathogenic T cells of origin in this disease. Utilizing high-throughput screening (HTS) of the T cell receptor (TCR) and immunostaining, we found that clinically resolved psoriatic lesions contained oligoclonal populations of T cells that produced IL-17A in both resolved and active psoriatic lesions. Putative pathogenic clones preferentially utilized particular V&bgr; and V&agr; subfamilies. We identified 15 TCR&bgr; and 4 TCR&agr; antigen receptor sequences shared between psoriasis patients and not observed in healthy controls or other inflammatory skin conditions. To address the relative roles of &agr;&bgr; versus &ggr;&dgr; T cells in psoriasis, we carried out TCR/&dgr; HTS. These studies demonstrated that the majority of T cells in psoriasis and healthy skin are &agr;&bgr; T cells. &ggr;&dgr; T cells made up 1% of T cells in active psoriasis, less than 1% in resolved psoriatic lesions, and less than 2% in healthy skin. All of the 70 most frequent putative pathogenic T cell clones were &agr;&bgr; T cells. In summary, IL-17–producing &agr;&bgr; T cell clones with psoriasis-specific antigen receptors exist in clinically resolved psoriatic skin lesions. These cells likely represent the disease-initiating pathogenic T cells in psoriasis, suggesting that lasting control of this disease will require suppression of these resident T cell populations.

A primary role for human central memory cells in tissue immunosurveillance

Central memory T cells (TCM) patrol lymph nodes, providing central immunosurveillance against known pathogens, but have not been described as conducting primary tissue immunosurveillance. We analyzed the expression of tissue-homing addressins in human TCM vs effector memory T cells (TEM) from the same donors. In humans, the majority of human TCM were tropic for either skin or gut, and the overall tissue tropism of TCM was comparable to that of TEM TCM were present in healthy, noninflamed human skin, lung, colon, and cervix, suggesting a role for TCM in the primary immunosurveillance of peripheral tissues. TCM also had potent effector functions; 80% of CD8+ TCM produced TC1/TC2/TC17/TC22 cytokines. TCM injected into human skin-grafted mice migrated into skin and induced inflammatory eruptions comparable to TEM-injected mice. In summary, human TCM express peripheral tissue-homing receptors at levels similar to their effector memory counterparts, are found in healthy human tissues, have impressive effector functions, and can act alone to induce skin inflammation in human engrafted mice. Our studies support a novel role for human TCM in primary immunosurveillance of peripheral tissues and highlight the important role of this long-lived cell type in tissue-based immune responses.

High-throughput sequencing of the T cell receptor β gene identifies aggressive early-stage mycosis fungoides

The malignant T cell clone frequency in cutaneous T cell lymphoma lesions is an independent prognostic indicator for early disease progression and death. Predicting progression in a T cell lymphoma Mycosis fungoides (MF) is an often indolent cutaneous T cell lymphoma identifiable by the T cell receptor gene TCRB in skin biopsies. de Masson et al. performed high-throughput sequencing of MF skin lesions to determine whether the frequency of the most abundant TCRB sequence could predict the fraction of cases that progress to more aggressive disease. This tumor clone frequency (TCF) outperformed commonly used prognostic indicators of disease progression and overall survival in MF, and was particularly useful in early-stage disease. The TCF may thus help identify which MF patients in the same clinical disease stage are in fact more likely to progress to life-threatening disease. Mycosis fungoides (MF), the most common cutaneous T cell lymphoma (CTCL) is a malignancy of skin-tropic memory T cells. Most MF cases present as early stage (stage I A/B, limited to the skin), and these patients typically have a chronic, indolent clinical course. However, a small subset of early-stage cases develop progressive and fatal disease. Because outcomes can be so different, early identification of this high-risk population is an urgent unmet clinical need. We evaluated the use of next-generation high-throughput DNA sequencing of the T cell receptor β gene (TCRB) in lesional skin biopsies to predict progression and survival in a discovery cohort of 208 patients with CTCL (177 with MF) from a 15-year longitudinal observational clinical study. We compared these data to the results in an independent validation cohort of 101 CTCL patients (87 with MF). The tumor clone frequency (TCF) in lesional skin, measured by high-throughput sequencing of the TCRB gene, was an independent prognostic factor of both progression-free and overall survival in patients with CTCL and MF in particular. In early-stage patients, a TCF of >25% in the skin was a stronger predictor of progression than any other established prognostic factor (stage IB versus IA, presence of plaques, high blood lactate dehydrogenase concentration, large-cell transformation, or age). The TCF therefore may accurately predict disease progression in early-stage MF. Early identification of patients at high risk for progression could help identify candidates who may benefit from allogeneic hematopoietic stem cell transplantation before their disease becomes treatment-refractory.