Adoptive cellular immunotherapy (ACT) is the transfer of ex vivo-cultured immune cells primed for antitumor reactivity into a tumor-bearing host. It takes advantage of the host’sown ability to destroy tumors by stimulating immune effector cells to better recognize and eliminate tumor cells. Many different types of immune cells have been used in ACT, including natural killer (NK) cells, tumor-infiltrating lymphocytes (TILs), lymphokine-activated killer (LAK) cells, and cytokine-induced killer (CIK) cells. Among them, CIK cells appear to be the most promising cytotoxic effector cell type.

CIK cells are a heterogeneous subset of T lymphocytes with NK functional properties. The qualifier “cytokine- induced killer” indicates that they are generated via administration of cytokines during in vitro culture Arafar, 2014Linn and Hui, 2010. Cells which have the most potential effector function in CIK culture coexpress CD3 and CD56 surface molecules; possess a potent, MHC-unrestricted tumor-killing ability; and significantly reduced alloreactivity Sangiolo et al., 2008. CIK cells recognize tumor cells through the binding of CIK NKG2D receptors with tumor cell ligands such as MHC class I polypeptide-related sequences A and B (MICA and MICB); they then facilitate tumor cytolysis through secretion of perforin and granzyme degranulators Mehta et al.,1995Pievani et al., 2011Schmidt-Wolf et al., 1993. CIK cells display greatly reduced alloreactivity across the MHC barrier, theoretically due to abundant production of IFN- γBaker et al., 2001. Finally, CIK cells secrete IL-2, IL- 10, IL-6, IFN-γ, and TNF-α, which promote cytotoxic immune cell activity; and do not secrete suppressors such as IL-4, IL-7, and IL-12.

Initially described by Schmidt et al. and further characterized by Ortaldo et al., CIK cells are capable of killing a wide range of tumor cells, and hold great promise in the field of adoptive immunotherapy for cancer treatment Ortaldo et al., 1991Schmidt et al.,1986Schmidt-Wolf et al., 1991. These cells can be amplified in culture media from bone marrow, peripheral blood, or umbilical cord blood upon timedependent addition of IFN-γ, anti-CD3 antibody, and IL-2, as well as other cytokines. After 21 to 28 days of culture, CD3⁺CD56⁺ cell expansion ranges from one hundred to one thousand fold Hoyle et al., 1998Lu and Negrin, 1994Schmidt-Wolf et al., 1991. Other studies have also shown that functional CIK cells can be differentiated from the MNCs of either fresh Durrieu et al., 2013Introna et al., 2006Zhang et al., 2015 or banked Pham et al.,2015 umbilical cord blood, and effectively used in combination with other treatments for, among others, the relapse of acute leukemia after cord blood transplantation Introna et al., 2010, advanced solid malignancies after first-line chemotherapy failure Niu et al., 2011, and esophageal cancer Wang et al., 2014.

However, a standard protocol for umbilical cord blood-derived CIK cell generation has not been established. Previous studies have shown that CIK cells can be generated effectively from UCB-MNCs in different complete growth media, but vary with regards to cytokine concentrations of anti-CD3 antibody and IL-2 following 1000 U/mL IFN-γ Introna et al., 2006Introna et al., 2010Schmidt-Wolf et al., 1991Zhang et al., 2014Zhang et al., 2015. We split commonly used concentrations into four different experimental groups and used RPMI-1640 complete medium to create four disparate environments for stimulation of our CIK cells. These induced cells were then analyzed for cell marker expression and transcription-level cytokine production to determine the optimum cytokine concentrations for CIK cell generation.

CIKs are a heterogeneous population of cells displaying a mixed T-NK phenotype, non-MHC-restricted tumor-killing activity, and low alloreactivity that hold great promise in the field of cancer immunotherapy.

Previous studies have shown that CIK cells can be generated via incubation of UCB-MNCs with various concentrations of specific cytokines. The classical protocol uses IFN-γ, anti-CD3 antibody, and IL-2, in a complete culture media. Concentrations of these basic cytokines vary across studies; the only common factor is the addition of 1000 U/mL IFN-γ to the culture at day 0. Concentrations of anti-CD3 antibody range from 25 to 50 ng/mL, and those of IL-2 from 300 to 1000 U/mL. Total culturing time also varies from 14 to 28 days Introna et al., 2006Introna et al., 2010Schmidt-Wolf et al.,1991Zhang et al., 2014Zhang et al., 2015.

In our study, we started with the classical protocol of generating CIK cells from the MNCs of three umbilical cord blood samples using RPMI complete medium, then applied four different protocols with regard to concentrations of anti-CD3 antibody and IL-2. All protocols began with the addition of 1000 U/mL IFN-γ to the culture at day 0, followed by supplementation of anti-CD3 antibody and IL-2 on day 1, for a total culture period of 21 days. Concentrations of 25 and 50 ng/mL of Anti-CD3 and 500 and 1000 U/mL of IL-2 were tested in different combinations.

In culture, the addition of IFN-γ prior to IL-2 supplementation has the effect of increasing cytotoxicity; supplementing these cytokines in the reverse order would decrease cytotoxicity Schmidt-Wolf et al., 1991. Anti-CD3 antibody enhances T cell differentiation Lu and Negrin, 1994Ochoa et al., 1987. IL-2 is required for cell proliferation and survival, since in the absence of CD3, cell expansion is greatly limited, and cell death occurs after approximately 7 days in culture Baker et al., 2001. Our results indicate that protocols 3 and 4, with 50 ng/mL of Anti-CD3 and 500 and 1000 U/mL of IL-2 (respectively) yield optimal CIK cell production.

As a heterogeneous population, there are two main subsets in CIK cells: those that are positive for the membrane expression of CD56 (CD3⁺CD56⁺) and those that are negative for this marker (CD3+CD56-). However, only cells which express both the T cell marker CD3 and the NK cell marker CD56 have been shown to display anti-tumor capabilities. Immunophenotyping the expanded cells using flow cytometry found CD3⁺CD56⁺ subpopulations of 26.08% and 26.21% in experimental groups 3 and 4, respectively, with the highest result being sample 2 in group 4 after a 14-day culture, with a 32.88% CD3⁺CD56⁺ subpopulation; these percentages are in accordance with previous studies on CD3⁺CD56⁺ CIK cell culture from cord blood samples Introna et al., 2006Zhang et al., 2014Zhang et al., 2015. After 14-21 days in the culture, the number of CD3⁺CD56⁺ cells increased by well over a hundred fold, also in accordance with prior reports, which demonstrate that these cells can be expanded up to one thousand fold from both cord blood MNCs Introna et al., 2006 and peripheral blood MNCs Hoyle et al., 1998Kim et al., 2007aKim et al., 2007bOlioso et al., 2009.

Once endowed with NK function, CIK cells also recognize MHC class I-related molecules (MICA and MICB) and members of the UL16 binding protein family (ULBPs) on malignant tumor cells upon interaction with their NKG2D surface receptors, which leads to perforin and granzyme degranulation and tumor cell lysis Mehta et al., 1995Pievani et al., 2011. The induced cells in protocols 3 and 4 both expressed high levels of granzyme B (82.23% and 81.96%). The heterogeneous populations of generated cells included the CD3⁺CD56⁺ CIK cells, CD3^-CD56⁺ NK cells, and CD3+CD8+ cytotoxic T lymphocytes; these all contribute to the antitumor activity of the bulk culture.

Another characterization of the antitumor response is the TH1 cytokine release of IFN-γ and TNF-α. IFN-γ, which is a cytokine employed during both innate and adaptive immune responses, produces downstream effects such as increased presentation of class I and II MHC molecules, activation of macrophages, differentiation of naive T cells into TH1 cells, and induction of B cell isotype switching Abbas et al., 2012Aittomäki,2003. TNF-α is a pro-inflammatory cytokine that induces fever and inflammatory responses, apoptosis, and tumor cell necrosis Wajant et al., 2003. At the transcriptional level, the expression of genes that produce these cytokines was higher in the experimental cell populations as compared to fibroblast control samples. Greater production of IFN-γ across MHC barriers can also benefit the preservation of grafts in the presence oftumors without resulting in graftversus- host disease, enhancing the clinical utility of CIK cells in cancer treatment Baker et al., 2001. CIK cells also secrete high levels of IL-2 via overexpression. IL-2 is an essential factor for growth, survival, and differentiation of T lymphocytes Abbas et al., 2012Malek and Bayer, 2004 including NK-T cells. Acting as an autocrine cytokine, IL-2 plays an important role in the generation and proliferation of effector CIK cells. A great deal of prior research has established that CIK cells secrete IFN-γ, TNF-α, and IL-2 Hoyle et al., 1998Kim et al.,2007bWang et al., 2008Zhang et al., 2015.

The cell marker analyses and cytokine production assays from the three different samples in each treatment group revealed that the cytokine concentrations in experimental groups 3 and 4 produced the best results. Both generated large numbers of CIK cells in the presence of anti-CD3 antibody at 50 ng/mL, varying only in IL-2 concentration (500 U/mL and 1000 U/mL, respectively). From a cost-effectiveness standpoint, the group 3 protocol would be more appropriate because of the lower IL-2 concentration used; however, group 4 has the advantage of generating effective CIK cells in a shorter 14-day time period. Recently, researchers have expanded upon classical protocols to attempt to improve CIK cell proliferation and efficacy with additional cytokine inducers such as IL-6, IL-7, IL-15, IL-21, thymoglobulin, or co-culture with dendritic cells Bonanno et al., 2010Heninger et al., 2012Lin et al., 2012Rettinger et al., 2012Schmidt et al., 2004Zhao et al., 2012. More research, including further delineation of CIK cell characteristics, is needed to see if different combinations of these innovative inducers with those outlined in experimental groups 3 and 4 in this study could generate even higher percentages of CD3⁺CD56⁺ CIK cells.

In conclusion, the optimal environment for generation of CIK cells from umbilical cord blood in RPMI complete medium is in the presence of IFN-γ at 1000 U/mL, followed 24 hours later by addition of 50 ng/mL of anti-CD3 antibody and either 500 U/mL of IL-2 for a 21-day culture period, or 1000 U/mL of IL-2 for a 14-day culture period. Our results showed high levels of granzyme B-positive CD3⁺CD56⁺ subpopulations using both of these protocols. Moreover, these cells demonstrated transcriptional overexpression of the three genes that produce the crucial cytokines IFN- γ, TNF-α, and IL-2. This research is the first to experimentally generate the most effective procedures for generation of CIK cells from umbilical cord bloodderived mononuclear cells, promoting further use of this promising vehicle for immunotherapy research and clinical applications.