Patients with low titers of anti-Hu, the paraneoplastic encephalomyelitis/sensory neuronopathy (PEM/PSN) antibody, have a better tumor prognosis that those who do not harbor these antibodies. Accordingly, we examined the effects of serum from patients with anti-Hu antibodies on human tumor cell lines, in order to determine: (1) if the serum was toxic (growth inhibition or cytolysis) to tumor cells with or without complement, and (2) if anti-Hu antibodies contributed to tumor toxicity. The serum of 14 patients with anti-Hu associated PEM/PSN, 22 patients with small-cell lung cancer (SCLC) without anti-Hu antibodies, and 20 normal individuals were studied. Three cell lines (NT-2, BE(2)-C, and SH.SY5Y) that express Hu proteins, and one cell line (SAOS-2) that does not, were studied. We examined the effects of whole serum, IgG-depleted serum, and purified IgG in the presence or absence of complement. A higher percentage of anti-Hu sera were toxic (71%) compared with sera from anti-Hu negative SCLC patients (23%) (p < 0.0001). No correlation existed between the titer of anti-Hu antibodies and toxicity. The toxic effects were observed in all tumor cell lines including the cell line that does not express Hu antigens. Toxicity persisted in serum depleted of IgG. Purified anti-Hu IgG in the presence and absence of complement, was not toxic. Our findings indicate that anti-Hu serum is toxic for human tumor cell lines, but this toxicity does not appear to be mediated by anti-Hu antibodies.

Antibodies against the HuD antigen expressed in small-cell lung cancer (SCLC) cross-react with proteins expressed in neurons of the central and peripheral nervous system and are associated with paraneoplastic encephalomyelitis and sensory neuropathy (PEM/SN). We isolated anti-HuD Fab fragments from an antibody phage display library that was constructed from mRNA of a metastatic lymph node from a patient with SCLC and Pem/SN. Fab GLN495 recognized HuD and other related proteins (HuC and Hel-N1, or Hu antigens) in immunoblots of these recombinant proteins and in immunohistochemical and Western blot analysis of SCLC and neurons. Fab GLN495 inhibited up to 75% of the anti-Hu antibodies of the patient from which it was derived, suggesting that recognizes a dominant epitope in the polyclonal anti-Hu antibody response. Fab GLN495 also competed with anti-Hu sera from most but not all patients with PEM/SN, indicating that the same epitope is recognized by a large subgroup of patients. Human monoclonal anti-HuD antibodies may be useful in diagnosis of HuD expressing tumors and in clarifying the autoimmune etiology of PEM/SN. This study, the first to demonstrate that tumor specific recombinant antibodies can be isolated from metastatic lymph node tissue, shows that this approach may be generally applicable to isolate human antibodies against tumor specific antigens.