Dynamic Changes in Macrophage Polarization Correlate with Progression and Response in CNS Lymphoma: Insights Into the Transcriptome of Lymphoma-Associated M2 Macrophages,

Macrophages are an important component of the tumor microenvironment and the immune response to malignancy. Classically activated (M1-polarized) macrophages exhibit anti-tumor effect, while alternatively activated (M2-polarized) macrophages promote tissue repair, angiogenesis, immunosuppression and tumor progression. To date, the vast majority of studies on macrophage phenotype and polarization have been based upon in vitro studies or murine model systems. We tested the hypothesis that distinct macrophage subsets can be identified within the microenvironment of lymphoid tumors and that the relative proportion of M1:M2 macrophages correlates with therapeutic response and/or resistance. In addition, we postulated that transcriptional analysis of distinct macrophage subpopulations would provide insights into mechanisms by which M2 macrophages promote tumor progression and reveal novel mechanisms which direct the interconversion of activated macrophages between classical and alternative activation states. Our focus has been on the evaluation of the tumor microenvironment in CNS lymphomas, the cerebrospinal fluid (CSF), which is enriched in inflammatory cells including activated CD14+ macrophages that we hypothesized would exhibit phenotypic features consistent with M1 and M2 polarization.

We developed a novel 9-parameter flow-cytometric method to isolate distinct subpopulations of activated macrophages from CSF and peripheral blood. The method identifies macrophages with M1 features based upon expression of nitric oxide synthase (iNOS) and M2 macrophages on the basis of low-expression of iNOS and high expression of scavenger receptors CD206, CD209 and CD36. Subpopulations of macrophages were quantified and analyzed by FACS and transcriptional profiling (Affymetrix Gene Chip 1.0 ST). Serial analysis of CSF was performed in CNS lymphoma patients to correlate macrophage polarization states with response and/or progression. Parallel analyses were performed in blood in patients with non-Hodgkin lymphoma (NHL) to control conditions including infections and/or sarcoid.

At least four distinct subpopulations of activated macrophages were identified in CSF in association with CNS lymphoma (N=30) and controls (N=28). A greater than six-fold increase in the proportion of M2 macrophages was detected in CSF of CNS lymphoma patients compared to control subjects (p<0.001). By contrast, the proportion of CSF macrophages with M1 features was similar between NHL and controls. Moreover, peripheral blood macrophages from CNS lymphoma patients did not demonstrate significant M2-polarization. Serial analysis demonstrated that M2 polarization of CSF macrophages correlated with tumor progression and/or response. Transcriptional profiling analysis was conducted on macrophage subpopulations isolated from CSF of CNS lymphoma patients and controls. While few significant transcriptional differences were identified between M1 and unpolarized (Mnaive) macrophages, the transcriptional comparison of M2 vs. M1 as well as M2 vs. Mnaive macrophage subpopulations revealed several hundred differentially expressed genes with significant adjusted p-values. Pathway Analysis (Ingenuity) suggested mechanisms strongly associated with the M2 phenotype including activation of Notch signaling. Transcripts associated with short survival were most strongly expressed by M2 macrophages and included Ephrin A4 and lymphoid chemokines such as CXCL-13. The prognostic significance of high CXCL-13 and Ephrin A4 expression were confirmed by ELISA in an independent validation set of cases.

We believe this to be the first application of flow-cytometry to define the phenotypes and dynamic interconversion of intratumoral macrophages within the lymphoma microenvironment and the first correlation of M2 macrophages with the evolution of resistance to therapy. In addition, this dataset provides the first in-depth transcriptional profiling analysis of in vivo human macrophage subpopulations and suggests novel mechanisms by which tumor-associated macrophages may facilitate lymphoma progression via the regulation of the metabolic microenvironment, angiogenesis, tumor invasion and immunosuppression. Based upon our additional preliminary data, we hypothesize that these results may be relevant to a variety of lymphoid tumors.

Off Label Use: We will discuss the use of rituximab within the leptomeningeal compartment to treat recurrent/refractory CNS lymphomas.