Discrepancies in the Tumor Microenvironment of Spontaneous and Orthotopic Murine Models of Pancreatic Cancer Uncover a New Immunostimulatory Phenotype for B Cells

Abstract

B cells are salient features of pancreatic ductal adenocarcinoma (PDAC) tumors, yet their role in this disease remains controversial. Murine studies have indicated a protumoral role for B cells, whereas clinical data show tumor-infiltrating B cells are a positive prognostic factor, both in PDAC and other cancers. This disparity needs to be clarified in order to develop effective immunotherapies. In this study, we provide new evidence that reconcile human and mouse data and highlight the importance of using relevant preclinical tumor models when assessing B cell function. We compared B cell infiltration and activation in both a genetic model of murine PDAC (KPC mouse) and an injectable orthotopic model. A pronounced B cell infiltrate was only observed in KPC tumors and correlated with T cell infiltration, mirroring human disease. In contrast, orthotopic tumors exhibited a relative paucity of B cells. Accordingly, KPC-derived B cells displayed markers of B cell activation (germinal center entry, B cell memory, and plasma cell differentiation) accompanied by significant intratumoral immunoglobulin deposition, a feature markedly weaker in orthotopic tumors. Tumor immunoglobulins, however, did not appear to form immune complexes. Furthermore, in contrast to the current paradigm that tumor B cells are immunosuppressive, when assessed as a bulk population, intratumoral B cells upregulated several proinflammatory and immunostimulatory genes, a distinctly different phenotype to that of splenic-derived B cells; further highlighting the importance of studying tumor-infiltrating B cells over B cells from secondary lymphoid organs. In agreement with the current literature, genetic deletion of B cells (μMT mice) resulted in reduced orthotopic tumor growth, however, this was not recapitulated by treatment with B-cell-depleting anti-CD20 antibody and, more importantly, was not observed in anti-CD20-treated KPC mice. This suggests the result from B cell deficient mice might be caused by their altered immune system, rather than lack of B cells. Therefore, our data indicate B cells do not favor tumor progression. In conclusion, our analysis of relevant preclinical models shows B cells to be active members of the tumor microenvironment, producing immunostimulatory factors that might support the adaptive antitumor immune response, as suggested by human PDAC studies.

Human Colorectal Carcinoma Infiltrating B Lymphocytes Are Active Secretors of the Immunoglobulin Isotypes A, G, and M

“…Tumor-associated B cells were enriched for activated and terminally differentiated B cells. Very recently, Spear and colleagues shed light onto the allegedly divergent pro- or anti-tumoral role of TiBc in a matched analysis of a genetic and an orthotopic cell injection model of murine pancreatic carcinoma [12]. They concluded that TiBc do not support tumor growth and are an active member of the tumor microenvironment in the genetic mouse model, which best reflects (slow) natural human tumor development.…”

Preclinical mouse models for immunotherapeutic and non-immunotherapeutic drug development for pancreatic ductal adenocarcinoma

“…Syngeneic orthotopic models are also better at representing the cell-to-immune system interactions. However, recent reports showed that the tumor infiltration B cells in syngeneic orthotopic models are significantly less than in genetically engineered KPC mice, indicating that two models are still different and therefore should be used together for immunotherapeutic development ( 102 ). For instance, using a combination of GEMMs and orthotopic models, Foley et al .…”

Pancreatic Adenocarcinoma Invasiveness and the Tumor Microenvironment: From Biology to Clinical Trials

“…B cells may promote tumorigenesis and proliferation of pancreatic cancer [ 103 , 113 ]. However, recent studies show that B cells may be immunostimulatory rather than immunosuppresive [ 114 ]. These contradictory functions for B cells in PDAC could suggest a dynamic role within the tumor much like other stromal components.…”