Hodgkin Lymphoma: Metabolic Symbiosis Between Malignant Cells and Cancer-Associated Stromal Tissue
Authors: Sprandio John D , Dulau Florea Alina E. , Martinez-Outschoorn Ubaldo 
Published: October 2019

Background
Hodgkin lymphoma (HL) is mainly composed of reactive cells. Lymphocytes, macrophages, eosinophils, mast cells, plasma cells, and other stromal cells constitute these supportive or reactive cells. In-situ glycolytic and oxidative phosphorylation metabolism of reactive cells and cancer cells is unknown in HL. We interrogated HL specimens (N=24) to examine the metabolic compartments in-situ in this disease.

Introduction
High glycolytic activity is seen in the majority of HL on the basis of high uptake of 2-[18] fluoro-2-deoxy-glucose (FDG) on positron emission tomography (PET). The advent of FDG-PET in HL has revealed that these tumors as a unit are glycolytic and staging and assessment of response to treatment has improved. However, it is unknown if cancer cells and reactive cells share a metabolic phenotype.

Purpose
To study mitochondrial and glycolytic metabolism in HL cancer and reactive cells.

Methods/Materials
Two immunohistochemical biomarkers of metabolism were employed on HL tumor sections. Translocase of the Outer Mitochondrial Membrane 20 (TOMM20) protein expression is a biomarker of functional mitochondrial mass and oxidative phosphorylation. Monocarboxylate transporter 4 (MCT4) is a biomarker of glycolysis and lactate export. We selected 24 consecutive cases of classical HL based on morphology and immunohistochemical (IHC) staining for CD30, CD15 and CD45. We recorded FDG-PET standard uptake values (SUV) where available for all tumors studied. Immunohistochemical stains were performed on 5-micron thick, formalin-fixed, paraffin-embedded tissue sections using the horseradish peroxidase method. The primary antibodies used were TOMM20 and MCT4 (Santa Cruz). The immunostaining was graded on a scale of 0 to 2+ according to the intensity and percentage of immunoreactive cancer and reactive cells.

Results
Reed Sternberg and Hodgkin cells (RS/H) had high TOMM20 expression and low MCT4. In fact, the highest TOMM20 expression out of all cells analyzed was found in the RS/H cells in 20 of the 24 samples. In the other 4 samples, cancer and reactive lymphocytes (RL) had similar expression of TOMM20. The tumor stroma or stromal cells in proximity to cancer cells which includes histiocytes (TS/H) had low TOMM20 expression and high MCT4 expression in all 24 samples analyzed. Higher than baseline FDG-PET uptake is a measure of glycolysis and was found in all tumors where FDG-PET was performed. High MCT4 expression was not found in normal stroma or stroma at a distance from cancer cells. Histiocytes always had low TOMM20 expression but MCT4 expression was high in proximity to cancer cells and low at a distance from cancer cells.

Conclusion
Glycolysis and lactate export occurs in cancer-associated stroma (TS/H) that is spatially linked to mitochondrial metabolism in cancer cells (RS/H) in HL. This suggests that the most FDG-PET avid cells within HL are the reactive cells and not the cancer cells. Also, normalization of FDG-PET uptake when assessing response to treatment suggests reversal to a metabolically normal stroma.
Representative images, 60x:

Disclosures:
No relevant conflicts of interest to declare.