Induced antitumor immunity against DMBA‐4 metastatic mammary tumors in rats using laser immunotherapy

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Abstract
Induced antitumor immunity is a highly effective and long‐term cure for cancer, particularly for metastatic tumors. Laser immunotherapy was developed to induce such an immunologic response. It involves intratumoral administration of a light‐absorbing dye and a specially formulated immunoadjuvant, followed by noninvasive irradiation of a near‐infrared laser. Treatment of DMBA‐4 metastatic mammary tumors in rats with this approach has resulted in local control of primary tumors and eradication of untreated distant metastases. After laser immunotherapy, rats were resistant to tumor rechallenge and developed immunity, which could be adoptively transferred. To better understand the immunity induced in this tumor model, immunization using freeze–thaw DMBA‐4 cell lysates was performed, followed by tumor challenge 21 days later. Tumor cell lysate immunization delayed the emergence of metastases but did not provide immunity against the tumor challenge. Also performed was surgical resection of primary tumors before the observation of metastatic tumors. Removal of primary tumors was unsuccessful at changing the course of tumor progression. Tumors re‐emerged at the primary sites, and metastases developed at multiple remote sites. In contrast, tumor‐bearing rats successfully treated by laser immunotherapy experienced tumor regression and eradication and developed strong resistance to repeated challenges by tumor cells of the same type. Our results show that laser immunotherapy could have potential for the treatment of metastatic tumors by inducing tumor‐specific, long‐lasting immunity.