Pancreatic cancer is the second most common gastrointestinal cancer in the United States with an estimated incidence of 53,070 cases in the year 2016 . Survival outcomes are poor with an estimated 5year overall survival rate of less than 5%. The lack of validated prevention strategies and screening tools for early detection contribute to diagnosis of disease at more advanced stages. Radiation is used in early and locally advanced pancreatic cancer with some instances of utility in metastatic cancer as well . Yet local recurrences rates are in the tune of 30-40% . Currently there is increased recognition that the patient’s immune system can be harnessed to improve treatment outcomes. In addition, RT has been found to have a Janus effect on immune system. It has been found to be immunostimulatory as well as immunosuppressive depending upon the dose, fraction size, tumor type and site of radiation. RT exerts its immunostimulatory effects by
a) Increased expression of Major histocompatibility antigen (MHC) on the tumor cells
b) Release of tumor associated antigens (TAA) leading to an in-situ vaccine effect
c) Enhanced expression of immunostimulatory signals like calreticulin, adenosine triphosphate (ATP), Heat shock proteins (HSP), high mobility group box proteins (HMBG )
d) Enhanced cross presentation of TAA by dendritic cells
e) Expression of concealed antigens like cancer testis antigen
f) Increases expression of Cluster of differentiation (CD) 95/Fas, intercelular adhesion molecule (ICAM) increasing sensitivity to effector T cells.
Downside to RT is that it can suppress immune system by depletion of hematopoietic progenitor cells in the lymphoid organs, upregulation of programmed death domain ligand (PDL1) and proportional increase in T regulatory cells [4,5].
In spite of advances in surgery, chemotherapy and radiation long term resistance to tumor recurrence has remained elusive.
Immune system has specificity and memory which is not seen with any other therapeutic modality. RT has the potential to increase the impact of the immune system by enhancing the expression of pro immunogenic molecules and in turn the immune system also compliments the response to RT. RT and immunotherapy together mediate their anti-tumor effects by a dynamic interplay of CD8+ cytotoxic T cells, CD4+ effector T cells and T regulatory cells modulation. Among the primary lymphoid organs, RT dose to bone marrow is well known to be immunosuppressive studied in the context of medulloblastoma, total body irradiation and pelvic malignancies [6,7] Dose to the secondary lymphoid organs like spleen has been reported recently as an independent predictor of increased mortality secondary to lymphopenia in pancreatic cancer.  Researchers from John Hopkins have shown that lymphopenia due to RT dose to circulating lymphocytes may also be a predictor of poor overall survival in gliomas .
These observations point to the fact that CD4 helper T cells, CD8 effector T cells, T regulatory lymphocytes and B lymphocyte proportions might be altered. Recent studies have shown that RT dose to circulating lymphocytes reduces CD4 helper T cells, CD8 cytotoxic T lymphocytes and proportional increase in T regulatory cells. This alteration in T cell equilibrium might be responsible for reduced immunological cell death and inferior overall survival (OS). The recognition that immune system plays a vital role in tumor surveillance and the advent of immunotherapy has brought the focus on adequate number of lymphocytes in the circulation. Reduced pretreatment lymphocyte count and reduced lymphocyte infiltration in pathologically resected specimens have been associated with poor disease-free survival (DFS) and overall survival (OS) in breast, rectal, glioblastoma multiforme, non-small cell lung cancer and in a vast majority of tumors.
CL are the cells that eventually infiltrate the tumors. Hence depletion in circulating pool of lymphocytes might lead to suboptimal treatment outcomes. Radiation induced lymphopenia (RIL) has been shown to be an independent predictor of inferior overall survival in gliomas, pancreatic cancer, lung, hepatocellular malignancies and inferior
progression free survival (PFS) in head and neck cancers
[8,10-14]. RIL has also been associated with poor response to
therapy and higher recurrence rates in cervical and bladder
cancer. Unintentional splenic irradiation in pancreatic cancer
has been postulated to affect disease outcomes. Splenic Dmean
of >9 Gy causes lymphopenia and this lymphopenia might lead
to reduced infiltration of cytotoxic lymphocytes into the tumor
leading to suboptimal tumor control . It has also been found
that there is a failure in the compensatory rise of IL-7 and IL-15
which are homeostatic cytokines that are responsible for optimal
level of lymphocytes in circulation . Hence, we propose that
supplementation of IL-7 or IL-15 may overcome post radiation
lymphopenia and enhance infiltration of lymphocytes into the
tumor leading to better tumor control.
IL-15 is a 15,000 Dalton cytokine with structural homology
with IL-2 with respect to CD 122 and gamma chain receptor. It
stimulates the generation of cytotoxic CD8+ T lymphocytes, B
cells and NK cells similar to IL -2. But in differs from IL -2 in not
stimulating T regulatory cell production and with no capillary
leak syndrome . Hence it can be used an immunostimulant
therapeutically. Researchers from Weil Cornell have done in
vivo studies on breast cancer bearing mice and found that RT+
intratumoral IL-15 produced the greatest survival advantage
compared to with either one alone. This effect correlated with
increase expression of CD8+T cells, CD4 effector to T regulatory
cells. In a similar experiment they also showed that NK cells in
addition to CD8 T cells play a critical role in mediating tumoricidal
effect of RT+IL-15. They hypothesized that NK cell are critical
mediators either by direct cytotoxic effect or priming CD8+ T
cells or a combination of both . Mathios et al evaluated the
effect of IL-15 on murine glioblastoma tumors treated with 10
Gy of RT by SRS.They found RT+IL-15 provided the most durable
tumor response with increase in CD8 +T cells and NK cells. But
artificial depletion of NK cells had no effect on tumor control
. This provides conflicting reports on role of NK cells in
mediating IL-15 syngersitic effect with RT.Nerverthless, IL-15
tends to increase cytotoxic CD 8+ T lymphocytes with no effect
on T regulatory cells, can be used for treating or preventing RIL.
The advantage of IL-15 is that it is known to imbibe antigen
memory to the T cells. The possibility of immunological cancer
antigen memory in preventing long term recurrence of disease is
an interesting possibility worth exploring.
The first human clinical trial of IL-15 in metastatic cancer was
published in 2015 and it was found to be safe. IL-15 increased
serum NK cells and CD8 memory T cells. The shortcoming of
IL-15 for clinical use is its short half-life and the relatively high
dose required to achieve the desired effect in animal models
. Hence IL-15 superagonist/IL-15RaSushi-Fc fusion complex
(IL-15SA/IL-15RaSu-Fc; ALT-803) has increased affinity for
immune cells and has potent NK and CD8+ T cell stimulation
even at lower therapeutic concentrations. ALT-803 has shown
to be 5-25 times increased in vivo biological action and exhibits
longer half-life . This fusion complex has been studied in
lymphoma, rat bladder cancer and murine glioblastoma models
[21-30]. ALT-803 could be used as a protein scaffold to create
IL-15-based tumor cell-specific molecules. This novel fusion
protein exhibits enhanced anti-tumor activity compared to
rituximab while maintaining IL-15 immunostimulant properties.
Its utility in pancreatic cancer and its possible synergistic effect
with radiation is an area worth exploration. We propose that
IL-15 based complexes can have multi-pronged effects in terms
of rescue tool for radiation induced lymphopenia, enhance the
immunological effects of radiation and produce immunological
memory to tumor antigens thereby producing a vaccine effect.