The invention relates to pharmaceutical compositions comprising α-Gal BOEL for use in treating patients with tumors. The invention also relates to methods of treating tumors using said compositions. The invention discloses that following intratumoral injection of α-Gal BOEL, binding of the natural anti-Gal antibody to de novo expressed tumoral α-Gal epitopes induces inflammation resulting in an anti-Gal antibody mediated opsonization of tumor cells and their uptake by antigen presenting cells. These antigen presenting cells migrate to draining lymph nodes and activate tumor specific T cells thereby converting the treated tumor lesions into in situ autologous tumor vaccines. This therapy can be applied to patients with multiple lesions and in neo-adjuvant therapy to patients before tumor resection. In addition to the regression and/or destruction of the treated tumor, such a vaccine will help in the immune mediated destruction of micrometastases that are not detectable during the removal of the treated tumor. The invention further teaches the enhancement of anti-tumor α-Gal BOEL treatment by the use of antibodies that inhibit the activity of immunological checkpoints molecules.