**4.3. Immunologic response to MWA**

The immunologic response to MWA is less well characterized compared to other methods of ablation, such as RFA and cryoablation. The vast majority of the research regarding the immune response and immunologic stimulation has been studied with either RFA or cryoablation. Recently specific immunologic mechanisms and the effects following MWA have been studied in more detail. It has been assumed that the immunologic response to MWA is similar to the mechanism and response to RFA [90, 91]. Currently, our knowledge about the immune response to MWA comes from both animal and clinical trials of various tumor types from breast to hepatic carcinomas [92–94].

In regards to patient management and future treatment, the goal of therapy is to generate a lasting immune response that results in regression of distant lesions and generate protection from disease recurrence. As detailed in previous sections, the aim of treatment is to generate specific cytokines triggering the Th1 response and activating the cellular immune system. Similar to other ablative techniques, MWA alone is not powerful enough to trigger the desired immune response, but holds potential with combination therapy [90, 91].

The specific immune response to MWA in patients with HCC has been analyzed by Zhang et al. In their study, 45 patients with HCC treated with MWA had peripheral blood analysis following treatment. The results showed significant increases in IL-12 and decreases in IL-4 and IL-10 [91]. These results are promising since IL-12 is involved in the differentiation of Th1 cells and generation of cellular immunity. Furthermore, patients showed decreased levels of IL-4 and IL-10, which are involved in activation of humoral immunity. While MWA alone is not enough to create a significantly different clinical response, the cytokine profile produced is advantageous.

Various combination therapies have been studied ranging from OK-432, immunotherapy, GM-CSF, and CTLA-4 blockade [92–94]. OK-432, also known as picibanil, is a low virulence mixture of *Streptococcus pyogenes* that has been used as an antitumor agent since 1975 [95]. OK-432 is able to induce pro-inflammatory cytokines and activate the T-cell–mediated immunity. Li et al. demonstrated that MWA and OK-432 used in combination resulted in prolonged survival and a strong immunologic response to rechallenge in a murine model of breast cancer. The results showed that a dominant Th1 response is generated. The cytokines IL-12, IL-2, and IFN-γ were significantly increased with no effect on Th2-type cytokines. Additionally, immunohistochemical analysis showed that a predominance of CD8+ T cells infiltrating the treated tumors.

Immunotherapy combined with MWA for the treatment of HCC has been investigated in both phase I and phase II trials [93, 96]. This combination was shown to increase the absolute number of circulating lymphocytes. When analyzed for specific subgroups, patients showed increased levels of cytotoxic subsets of T cells and decreased suppressive subsets. Additionally, patients treated with immunotherapy had significantly improved liver function. However, the disease-free survival and overall survival rate were not significantly improved.

CTLA-4 blockade holds great promise when combined with cryoablation, but could also be used in with MWA. CTLA-4 blocking antibodies and GM-CSF combined with MWA were shown to induce tumor-specific cellular immune response in a murine model [94]. The combination of the three resulted in a 90% rejection upon tumor rechallenge and 50% of the animals treated showed distant tumor regression. Since both of these drugs are currently available for human use, this combination represents one that could be clinically used today.
