Even when it is diagnosed before the onset of overt metastatic disease, pancreatic cancer is inherently resistant to treatment and follows an aggressive clinical course. Intense desmoplasia, a hallmark of these tumors, and the high proportion of immunosuppressive cells in the pancreatic tumor microenvironment also contribute to the suboptimal outcomes.2 Emerging immunotherapy strategies in other disease sites are also being considered in pancreatic cancer to overcome this immune-privileged tumor microenvironment. Radiation therapy (RT) has a dual effect on the immune system. It can induce an in-situ vaccination effect via release of autoantigens and radiation-induced neoantigens, increase expression of co-stimulatory immune molecules on the surface of cancer cells, and express hallmarks of immunogenic cytotoxicity leading to enhanced cell death.3 But RT can also be immunosuppressive by inducing tumor overexpression of MHC-1, death receptors, and checkpoint proteins that drive co-inhibitory pathways to evade immune eradication;4 depleting lymphocytes in circulation and/or those sequestered in secondary lymphoid organs;5 inducing lymphocyte apoptosis via secretion of galectin-1 by tumors;6 and inducing TGF-β secretion that impedes the ability to generate an effective cytotoxic T cell response to tumor antigens.7
Researchers from John Hopkins have shown that post-treatment lymphopenia is correlated with increased morality in resectable and locally advanced pancreatic cancer. RT toxicity to circulating lymphocytes was postulated as the likely cause of lymphopenia that was independent of chemotherapy usage. Mortality was reported to be due to tumor progression rather than lymphopenia-related opportunistic infections.8,9 The spleen is not routinely considered a dose limiting organ and is commonly included in the RT portal in the treatment of pancreatic cancer. The spleen is a rich reservoir of T and B lymphocytes with a very slow circulation time due to the sinusoidal architecture of flow channels. Splenic dose beyond 10 Gy causes eventual stromal fibrosis leading to a dysfunctional spleen.10 The net effect is reduced levels of CD8+ cytotoxic T lymphocytes and CD4 + helper T cells available for tumor infiltration. Surgical resection specimens have shown that tumor infiltration of CD8+ T cells is associated with improved survival outcomes.11,12 Our publication showed that dose to the spleen (mean dose exceeding 9Gy and V15 above 20%) is an independent predictor of post-chemoradiation lymphopenia in locally advanced pancreatic cancer patients and that post-chemoradiation lymphopenia was strongly associated with poorer survival outcomes.13 Initial analyses noted that post-chemoradiation lymphopenia was not related to tumor size and therefore, presumably field size since patients were treated with local fields primarily. Subsequent analyses noted that both the irradiated volume and the treated volume (encompassed by the 50% and 95% isodose lines, respectively) were also not predictive of lymphopenia suggesting that depletion of circulating lymphocytes may not be a dominant mechanism of developing lymphopenia. If the splenic dose is independently validated in other studies to be a determinant of lymphopenia, a known driver of poor outcomes following chemoradiation, then there is a compelling rationale for triaging treatment plans based on their ability to adequately spare the spleen especially in patients with baseline lymphopenia prior to initiating chemoradiation, patients who are candidates for immunotherapy, and patients being considered for dose-escalated RT protocols.14 In addition to choosing spleen-sparing beam angles for 3D conformal RT, effective spleen sparing can also be accomplished by employing intensity modulated radiation dose painting, charged particle therapy, stereotactic body radiation therapy and high dose rate RT.15,16 More refined normal tissue complication probability (NTCP) modeling may also serve as a predictive tool to correlate fractional radiated volumes of the spleen (dose volume histograms (DVH)) with lymphopenia.
Similar to pancreatic cancer, post-RT lymphopenia has been correlated with inferior survival outcomes in lung cancer and high grade gliomas. In lung cancer, the greatest correlation between treatment parameters and post-chemoradiation lymphocyte nadirs was gross tumor volume (GTV) with larger GTVs causing lower lymphocyte nadirs.17 In high glioma patients, treatment-induced lymphopenia was again a poor prognostic factor for overall survival.18 In addition to the lymphotoxic effects of temozolomide and corticosteroids, lymphopenia was attributed to lymphotoxic radiation doses to circulating blood which in turn correlated with planning target volume (PTV) size but not radiation delivery technique.19 Furthermore, the lymphopenia failed to trigger a compensatory increase in interleukin-7 (IL-7) and interleukin-15 (IL-15), the key homeostatic cytokines that mediate the recovery of CD8+ cytotoxic T cells and CD4+ helper T cells.20
Accumulating evidence of a synergy between immunotherapy and radiotherapy has led to a flurry of clinical trials being planned for the treatment of pancreatic cancer. Currently there are over 30 clinical trials evaluating immunotherapy in pancreatic cancer. Optimal levels of CD8+ T cells and CD4+ helper cells a prerequisite for immunotherapeutic agents to act.21 Therefore, in addition to strategies that create an immune-permissive milieu within tumors via blockade of TGF-β and checkpoint signaling, treatments that directly increase lymphocyte counts could amplify the synergy and thus prevent or overcome the immunosuppressive effects of radiation. This immune-mediated tumoricidal effect could be effective locally within the tumor and/or distantly at sites of metastatic disease via long-term immunological memory of tumor associated antigens. Approaches being explored in this realm include IL-7 or IL-15 supplementation during RT, restitution of lymphocytes with autologous transfusion, and inhibition of galectin 1-mediated lymphocyte apoptosis possibly with thiodigalactoside. Preclinical models have demonstrated that administration of IL-7 to irradiated mice results in a preferential expansion of CD8+ cytotoxic T cells rather than T regulatory cells.22 Similarly, IL-15 generates a durable antitumor response when combined with RT, long-term immune memory against tumor rechallenge and improved survival.23,24 Currently there is one phase 1 clinical trial exploring IL-7 supplementation in high grade gliomas treated with RT.25 IL-15 is being evaluated as monotherapy in metastatic melanoma and renal cell carcinoma.26 Lymphocyte restitution is frequently utilized as a rescue strategy following myeloablative chemoradiation therapy in transplant protocols and adoptive T cell therapy.27 In one study, autologous lymphocytes harvested before temozolomide-radiation for high grade gliomas were reinfused after completion of treatment. The return of lymphocyte counts following radiation-induced depletion was, however, no different between reinfused patients and matched controls.28 Preclinical studies suggest that intratumoral thiodigalactoside administration prior to radiation rescued circulating CD8+ T cell and CD4+ T cell counts that were depleted by radiation.7
A preponderance of evidence suggests that lymphopenia confers a poor prognosis in the treatment of cancer.5,8,9,13,17,29-31 Coupled with this evidence, the recent illustration of a direct correlation between dose to critical normal tissues in radiation treatment plans and post-treatment lymphopenia in multiple tumor types is a harbinger of concerted efforts to prevent or overcome this detrimental effect of radiation. This can be achieved by selectively modifying treatment plans and delivery techniques (beam arrangements, intensity modulation, stereotactic radiation, proton therapy, and high dose rate), in scenarios where secondary lymphoid organs receive large doses of radiation that could contribute to lymphopenia, or replenishing and/or protecting lymphocytes from damage mediated by radiation therapy where the dose of radiation to the circulating blood pool (within the tumor or surrounding normal tissue) is the predominant cause of lymphopenia.
Written by: Bhanu Venkatesulu, Lakshmi Shree K Mahadevan, Maureen L. Aliru, Monica H. Bodd, Awalpreet S. Chadha, Steven H. Lin, and Sunil Krishnan.
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