The 9 patients enrolled for phase I were vaccinated 6 times, once every 2 weeks: 3 patients with 100 g and 3 patients with 300 g CHP-MAGE-A4, and 3 patients with 300 g CHP-MAGE-A4 plus 0.5 clinical units of OK-432. and 3 antibody responses were observed in 7/9 patients. Among them, positive conversions to T helper 2 (Th2)-type antibody responses (IgG4 and IgE) were observed IMR-1A after frequent vaccination in 4/7 patients. The Th2 conversion was possibly associated with undesirable clinical observations, including progressive disease and the appearance of a new relapse lesion. The present study suggested that frequent vaccinations activated a Th2-dominant status in the malignancy patients. The identification of a time-dependent IgG subclass and IgE antibody production during vaccination protocols may be a useful surrogate marker indicating a potentially undesirable change of the immunological environment for an effective antitumor immune response in malignancy patients. reported that IgG4 subclass antibodies impair antitumor immunity in melanoma (7). So there is a focused negative effect induced by EPHA2 IgG4 around the antitumor immune response. There have been few studies regarding the IgG subclasses and IgE during malignancy vaccination. To the best of our knowledge, the present study is the first to evaluate the time-dependent transition of the IgG subclass IMR-1A and IgE during malignancy vaccination. In this study, the CHP-MAGE-A4 vaccine induced mainly the Th1-dominant antibody response of IgG1, 2 and 3 production. However, positive conversions to the Th2-dominant antibody response designed that IgG4 and IgE were also observed after several rounds of vaccination in patients who previously had been positive for Th1-dominant antibody responses. IMR-1A In total, 3 PD and 1 SD clinical responses were observed in patients who showed the Th2 conversion in the antigen-specific antibody response, while there were 2 PD and 3 SD clinical responses in patients without Th2 conversion. These results suggest a possible association between the time-dependent Th2 conversion and the clinical benefit to the patient, although this issue must be rigorously confirmed in later stages of clinical trials aiming to address clinical response in a stringent manner with larger enrollment. Although it is unknown whether the reaction of the Th2-dominant antibody response depends on frequent medication or time after the first medication or IMR-1A superfluous Th1 reaction, in the present study, the rise in IgG4 antibody titer was delayed compared with the IgG1 response after frequent vaccination, confirming similar findings of a past study (5). IgG4 and IgE antibody responses were positive in patients 5 and 7, who had vigorous IgG1, 2 and 3 responses. These data suggest that a robust Th1-dominant antibody response may lead to conversion from a Th1 to a Th2 cytokine environment. By contrast, patient 4, who was mildly positive for a Th1-dominant antibody response, had only an IgG4 antibody response, and prolonged survival. However, this patient developed a new lesion, rising levels of tumor marker and an IgG4 antibody response at the same time, suggesting that the IgG4 antibody response may be a sensitive surrogate marker of undesirable change in the antitumor immune response. The current data showed that several injections of cancer vaccine were safe, but may cause an allergic reaction that is undesirable for creation of cancer immunity due to the similarity to conditions created during hyposensitization therapy for allergies. In past studies, self-antigen-derived cancer vaccines elicited allergic reactions. Moreover, the allergic reaction resolved after elimination of specific amino acid sequences known to evoke an allergic reaction from studies of the peptide involved (35,36). If characteristics of the IgG4 and IgE epitopes of MAGE-A4 were clarified, it would be possible to avoid an allergic-like reaction by the elimination of the pertinent IgG4 and IgE epitopes from the vaccine agent. In conclusion, the current results suggest that clinicians should be aware that frequent vaccine administration may induce a Th2 cytokine environment, and that there is a possibility that the IgG subclass and IgE antibody IMR-1A responses are useful as surrogate markers for an undesirable change in antitumor immunity, providing an indication to discontinue vaccine administration. Monitoring the time-dependent transitions of the IgG subclass and IgE levels will be important during cancer vaccination therapy. It may be necessary to reconsider protocols requiring frequent vaccinations at relatively short intervals. Patient sera from past cancer vaccine trials will aid in precisely addressing this possibility and also in clarifying the precise immunological mechanisms of the Th2 transition.