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2831 Poster Board II-807 Its tumor-restricted expression and its high immunogenicity render cancer-testis (CT) antigen NY-ESO-1 an exquisite target for antigen-specific immunotherapies. Spontaneous antibody responses against NY-ESO-1 are typically found in a subset of patients with solid tumors. However, little is known regarding serological immune responses against NY-ESO-1 in patients with hematological malignancies including multiple myeloma (MM). Furthermore, no consequent longitudinal analyses have been performed correlating NY-ESO-1-specific antibody titers with the clinical development of the given malignancy. Finally, nothing is known regarding the functional capabilities of spontaneously occurring anti-NY-ESO-1 antibodies in MM or other malignancies. Here, we performed the first longitudinal and functional investigation of NY-ESO-1-specific antibody responses in MM analyzing 1100 sera and 200 bone marrow plasma samples of 194 MM patients. Sera and BM plasma samples of 100 healthy donors served as controls. Screening sera and bone marrow plasma of our MM patients by Enzyme-linked Immunosorbent Assay (ELISA) using full length recombinant NY-ESO-1 protein, we found that 5/194 patients had high-titered antibody responses against this CT antigen. A quantitative B cell ELISPOT demonstrated NY-ESO-1-specific B cells in the peripheral blood as well as in the bone marrow of the respective MM patients. In a western blot analysis, spontaneous NY-ESO-1-specific immune responses in the patients were found to be highly specific for both native and recombinant protein. Epitope mapping in an ELISA using 18 overlapping NY-ESO-1 20mer peptides showed that antibody responses were restricted to the first 70 amino acids of the full-length protein. NY-ESO-1-specific antibodies consisted mainly of IgG1 and to a lower extent of IgG3 subtypes. No IgG2, IgG4, IgM or IgA antibodies against NY-ESO-1 were detected. Interestingly, antibody affinity increased over the course of the disease suggesting an affinity driven antibody maturation. Accordingly, NY-ESO-1-specific antibodies of MM patients were found to be potent complement activators in a western blot technique. On the other hand, despite the high functional capabilities of NY-ESO-1-specific antibodies, antibody titers increased with each NY-ESO-1-expressing (as indicated by reverse-transcriptase-polymerase-chain-reaction and immunohistochemistry) recurrence of the disease. In conclusion, we demonstrate here the spontaneous occurrence of high-titered NY-ESO-1-specific antibodies in MM patients. One reason for the relatively low frequency of antibody responses against NY-ESO-1 might be that most patients were in early stages of the disease or in remission at the time the analysis was performed. Antibodies were produced by NY-ESO-1-specific B cells detectable in the bone marrow as well as in the peripheral blood of the patients. NY-ESO-1-specific antibodies were evoked by a distinct and immunodominant fragment of NY-ESO-1. Affinity maturation of this response and complement activation by the spontaneously occurring NY-ESO-1-specific IgG1-type antibodies speak in favour of an effective serological immune response. However, positive correlation of antibody titers with tumor burden and recurrence of the disease suggest an inability of antibodies targeting intracellular protein NY-ESO-1 to control the course of the disease, at least in the long run. Antigen-specific immunotherapy might be necessary to shape NY-ESO-1-specific immunity in MM patients and, particularly, to mobilize tumor-specific T cell responses. Disclosures: No relevant conflicts of interest to declare.