INTRODUCTION

 The role of disturbed immune balance in the prognosis of patients with malignant tumors Within the last decade, there has been increased attention focused on the prognostic significance of immune status in malignancy. Specifically, an altered balance in the innate immune system has been identified as an important clinical predictor. Decreased activity of phagocytes and natural killer cells were identified as important features of cancer progression. Several studies were undertaken to characterize this tumor-related immune imbalance identifying important mechanisms. These include disturbed balance between Th1 and Th2 immune responses and increased activity of regulatory T cells and myeloid-derived suppressor cells (Nagtegaal et al., 2001; Ostrand-Rosenberg and Sinka, Advancement in Medicinal Plant Research Vol. 1(3), pp. 56-65, July 2013 Review 2009; Mantovani, 2007; Terabe and Berzofsky, 2008). Numerous studies have targeted the alterations of antitumor cellular immune response in various types of malignancy. In these studies, soluble factors were identified that are either produced or affected by malignant cells. 

In addition, enhanced suppressor activity of several lines of immunocompetent cells was also found to be an important factor in the development of the tumorrelated immunocompromised state (Baskic et al., 2001). It is now generally recognized that immunocompetent cells of the innate immune system are committed in two separate directions: (1) M1 macrophages and CD1a+ dentritic cells (DC1) generating IL-12 and other proinflammatory cytokines activating cytotoxic effector cells such as natural killer (NK) and natural killer T (NKT) cells, potent inhibitors of tumor growth; (2) prototypic M2 macrophages generating IL-4 and IL-10, which facilitate the generation of Th2 cells while inhibiting Th1 cells. Both of these two branches of innate immunity are affected by malignancy. The basic function of macrophages of the M2 type is to induce inflammation and promote cell proliferation by producing growth factors and through initiation of the arginase pathway, neoangiogenesis and tissue repair (Mantovani, 2007). In the presence of malignancy, a higher than normal proportion of macrophages belongs to this prototypic M2 population and this appears to alter the balance of immune responses (Sánchez-Torres, 2001). In fact, while in healthy individuals M2 monocytes comprise only 10% of the total monocytic population, this proportion in tumor patients may be as much as 40% higher (SánchezTorres, 2001).

 In malignancy, natural killer T cells (NKT) may have a protective effect by producing interferongamma, which activates both M1 and DC1 dendritic cells. These latter cells secrete IL-12, thereby increasing antitumor activity. On the other hand, NKT-2 cells inhibit tumor immunity (Terabe and Berzofsky, 2008). These opposing effects of different NKT cell lines provide a further example of the complexity of immune imbalance in cancer patients. While numerous studies employing different immunological techniques failed to provide evidence for a major role of T-cells in general in anti-tumor immunity (Nagtegaal et al., 2001), other studies specifically focusing on effector cells had more positive results. In particular, NK cells were identified as key players in eliminating tumor cells. Additionally, the administration of cytokines inducing natural killer cell response in combination with chemotherapy had additional benefits, including slowing tumor progression and improving survival (Wang et al., 2013). One must also remember that an important mechanism by which malignancy induces immunosuppression is through the induction of Treg cell proliferation as well as proliferation of other cell lines such as type-2 dendritic cells, which inhibit NK cell activity by decrease production of IL-12 (Mantovani, 2007). 

Hajtó et al. 57 Effect of various types of oncological treatments and interventions on the tumor-induced immune imbalance Cytostatic chemotherapy is widely used in various types of malignancy. However, with this therapy, complete remission is sometimes difficult to achieve, depending on the type of cancer. The effect of cytostatic drugs is not confined to malignant cells; they affect healthy cells among them immunocompetent cells. It seems therefore important to assess the influence of routine chemotherapy on tumor-related immune dysfunction. As far as mechanisms, many of the cytotoxic drugs can reduce the NK cell population, decrease IL-2 production and diminish antibody dependent cell-mediated cytotoxic (ADCC) activity. The suppression of the immune response reaches its peak in 3-4 weeks, with potential partial restoration of immune activity after 5 to 6 weeks (Saijo et al., 1982; Rafique et al., 1997; Kikuchi et al., 1997). The immunosuppressive effect of surgery is widely recognized. Postoperative activity of NK cells is significantly lower compared to preoperative values, and this effect lingers postoperatively for a long period of time (Bruns et al., 1996; Vallejo et al., 2003). The imbalance of the immune response in cancer patients is also associated with common tumor-related symptoms including fatigue and pain. When either of these symptoms are present, decreased activity of phagocytes and NK cells are often seen (Welden et al., 2009; Miller et al., 2008).

 This in turn leads to an alteration in the balance between type-1 and type-2 responses with elevated activity of regulatory T cells (Liu et al., 2009). This raises the possibility that analgesic treatments may be beneficial in these patients. Opioids would be the most logical candidates, except for the fact that they may have a more direct effect on the immune system on their own. When healthy volunteers were exposed to a morphine drip for a 36 h time period, this was associated with a suppression of both NK and phagocytic functions, confirming a direct effect of morphine on immunity (Yeager et al., 1995; Clark et al., 2007). Considerations for establishing a clinical strategy to improve anti-tumor cellular immune response through immunomodulatory treatments Although it is well established that cellular immunity is suppressed in malignancy, an effect further worsened by chemotherapy, this fact is not routinely considered in current clinical practice and did not inspire sufficient clinical research. This has led to the current state of affairs where insufficient experience and overall lack of understanding of the particular mechanisms prevent us from investing in studies to establish the worth of immunomodulatory treatment. We argue that if a clinical tool was available to test the degree and specific pattern of immunocompromise in cancer patients, this could open the way for an individualized immunomodulatory treatment of immune imbalance in these patients. One should also keep in mind the different mechanisms through which the two main types of cellular immune responses affect tumor growth. 

While in cancer patients, type-1 cells are activated inducing immune reactions that result in the eradication of tumor cells, eventually type-2 cells also become involved promoting tumor growth. If one was to devise an immunomodulatory therapy with the objective of boosting cellular immunity against malignancy, one should know exactly where an individual patient stands in the evolution of immune imbalance (Yeager et al., 1995). PROMISING PRECLINICAL AND CLINICAL RESULTS AND PERSPECTIVES OF IMMUNOMODULATORY TREATMENTS Restoring immune imbalance-what do we know from clinical studies? More than twenty years ago after the discovery of interleukin (IL)-2, Lymphokine Activated Killer cell (LAK) therapy was introduced, which seemed to achieve surprisingly rapid tumor remissions. It was soon realized that the activation of NK cells played the primary role in these successful treatments. In a recent paper, Levy and Roberti discussed the role of natural killer cells in human cancer (Levy et al., 2011; Roberti et al., 2012). As is well known, NK cells are some of the most important effector and regulatory cell populations of the innate immune system. As a result of this recognition, immunological research was for a long while primarily focused on NK cells in anti-tumor defense. Unfortunately, IL-2 is a highly toxic cytokine and its use produced a number of undesired side effects which considerably lowered enthusiasm for any further development of LAK therapy. 

In addition, it was also established that the activation of NK cells is not always accompanied by enhanced tumor cell killing- this same phenomenon was also observed during interferone therapy. Today, we can safely conclude that the role of NK cells in the rather complex network of innate immune system was overemphasized and the complexities of immune imbalance were poorly understood. It is small wonder that effective treatments could not be devised with such spotty understanding of mechanisms. It is well known that when activated, NK and Th1 cells are important interferone-gamma producers and by this effect they can induce enhanced expression of MHC-I antigens on the cell membrane of every nucleated cell. Adv Med Plant Res 58 Immunomodulatory treatments can enhance this effect. However, though increased MHC-I expression can be favorable for T cell-mediated adoptive immune reactions, it can also induce unfavorable mechanisms since MHC-I antigens on tumor cells can operate as Killing Inhibitor Receptors (KIR). Increased expression of KIR receptors on the surface of tumor cells may result in decreased efficacy of natural killer cells on these tumor cell lines. One is well advised to bear in mind this phenomenon, particularly when treating malignancy of a more advanced stage. In this circumstance, the relative role of the effector cells belonging to the innate immune system takes over the importance of adoptive responses (Nagtegaal et al., 2001; Clark et al., 2007) compromising efficacy of such therapies at this stage. 

In one study, for instance, when the relative significance between specific adaptive immune responses and non-specific reactions of innate immunity were compared and their relative contribution to prognosis analyzed, results revealed that although there was a significant anti-tumor response by the adaptive immune response as reflected by increased T cells activation, patient survival and tumor recurrence was affected more by non-specific responses by the innate immune system (Nagtegaal et al., 2001). However, persistent activation of effectors of the innate immune system such as NK, NKT and T cells together with Th1 cells can also upregulate KIR receptors on tumor cells by enhanced production of interferone-gamma. Consequently, the cellular cascade mechanisms against tumor cells will only be effective for a relatively short period of time and immunomodulators treatments need to be given periodically with therapy-free intervals of at least 72 h inserted between treatments. This was previously demonstrated in an animal model (Hajtó et al., 1998). To summarize, we need to devise our immune therapy in such a way that while we induce effector mechanisms by innate immunity, we also need to achieve periodic downregulation of MHC-I expression (Garrido et al., 1993; Lotfi et al., 2009). Subsequently, the clinical benefit of the down-regulation of MHC antigens by fermented wheat germ extract will be detailed. Can we enhance the effectiveness of monoclonal antibodies by immunomodulators? Interaction of cetuximab with NK cells As published previously and discussed subsequently, a combination of plant immunomodulators with Avastin (VEGFR inhibitor) induced a complete remission of hepatic metastases (Hajtó and Kirsch, 2013). 

As part of a complex regimen of oncotherapy, monoclonal antibodies (mAbs) have a central role in individualized cancer treatment. Recently, several studies undertook to further our understanding on monoclonal antibodies as they activate and/or enhance tumor-specific immune responses. It appears that these antibodies act either by interrupting intrinsic signal transduction cascades inside tumor cells, or by specifically delivering toxins to malignant cells as they block the tumor-stroma interaction (Vacchelli et al., 2013). Even though most studies focus on these aforementioned specified mechanisms by mAbs, monoclonal antibodies also have immuno-modulatory properties and they can activate the secretion of several substances crucial in the development of the immune response (Levy et al., 2011). These functions are less studied and remain poorly understood. Cetuximab, a monoclonal antibody, binds to the extracellular domain of EGFR, an antigen overexpressed in many different types of human cancers (Bou-Assaly and Mukherji, 2010). These types notably include head and neck and colorectal cancer. This process prevents EGFR from binding its endogenous ligand, blocking the receptor-dependent transduction pathway. This results in an anti-tumor effect due to a number of mechanisms including cell-cycle arrest, induction of apoptosis, inhibition of angiogenesis, inhibition of metastasis, internalization and downregulation of EGFR and increased sensitivity to radiotherapy and chemotherapy (Hidvégi et al., 1999). Levy et al. (2009) and Correale et al. (2010) showed that NK cells produce cetuximabmediated ADCC in metastatic colorectal cancer (mCRC). 

They also demonstrated that this activity is not affected by the mutational status of the molecule K-RAS. As they correctly state in their paper, this effect confirms previous observations that NK cells recognize the surface-bound Abs and are able to kill tumor cells independently from EGFR pathway activation. However, the failure of catuximab-mediated ADCC to induce clinical remissions in mCRC patients with K-RAS mutations remains largely unexplained. One of the possible factors involved could be the low proportion of NK cells infiltrating CRCs and the low functional capacity of these cells in cancer patients (Levy et al., 2009; Pagès et al., 2009; Wulff et al., 2009). If it was possible to enhance the activity of NK cells, the efficacy of cetuximab treatment as well as treatment with other mAbs (e.g. bevacizumab) could be improved. Furthermore, passive immunizations such as the ones affected by mAbs can also suppress innate cellular immunity by worsening cancer-related dysfunction. PROMISING PRECLINICAL AND CLINICAL RESULTS WITH PLANT IMMUNOMODULATORS Early results with immunomodulation therapy and clinical studies The immunomodulatory effect of mistletoe therapy, one of the better-studied forms of immunomodulation therapy, is based on lectin-carbohydrate interaction on cell membrane surface of several immunocompetent cells of Hajtó et al. 59 the innate immune system (Hajtó et al., 1989; Hostanska, 1996-97; Hajtó et al., 1990; Hajtó et al., 1998).