Clinical significance of anti-Hu antibody Anti-Hu antibody is an autoantibody against the nucleus of neuronal cells, and Hu antibody test is of clinical significance for the diagnosis of paraneoplastic neurologival syndrome (PNS), especially for the diagnosis of small cell lung cancer combined with paraneoplastic syndrome with high specificity and sensitivity [1]. It is particularly specific and sensitive for the diagnosis of small cell lung cancer combined with paraneoplastic syndrome [1]. At present, few neurologists in China have performed this antibody test and applied it to clinical diagnosis. The neurology laboratory of Peking Union Medical College Hospital introduced the experimental method of anti-Hu antibody test by immunofluorescence and protein immunoblotting at an early stage in China, and is one of the major laboratories in Beijing and China for anti-Hu antibody test. The following describes the anti-Hu antibody test method and analyzes the clinical significance of anti-Hu antibodies. I. Data and methods 1. Subjects for antibody testing: Venous blood and cerebrospinal fluid specimens from more than 1500 patients with various neurological diseases sent to the neurology laboratory of the Department of Neurology, Peking Union Medical College Hospital were tested (the test items were open and the clinicians determined the indications for antibody testing). 2. Experimental methods for antibody detection: Indirect immunofluorescence and protein immunoblotting methods were used. The indirect immunofluorescence method uses the anti-Hu antibody test kit from Eumont Experimental Immunoproducts (Germany) 3. Clinical review and analysis of patients with positive anti-Hu antibodies: The clinical data of our inpatients with positive anti-Hu antibodies were retrospectively analyzed, including the main types of clinical manifestations, the final diagnosis and the associated tumors. Diagnostic criteria for neurological paraneoplastic syndrome: Referring to the diagnostic criteria of Graus et al. for neurological paraneoplastic syndrome, the diagnosis of neurological paraneoplastic syndrome was confirmed by having any one of the following two items: 1) clinical manifestations of classical clinical syndromes (including subacute cerebellar degeneration, limbic encephalitis, subacute sensory neuronopathy, Lambert-Eaton syndrome), and anti-Hu antibody positivity, regardless of whether malignancy is found. (2) Clinical manifestations were not in accordance with the classical clinical syndrome, positive anti-Hu antibody, and malignant tumor was found. Results 1. Anti-Hu antibody test results: A total of 27 patients were positive for serum and/or cerebrospinal fluid anti-Hu antibodies, of which 26 cases were tested for serum anti-Hu antibodies and 25 cases were positive; 12 cases were tested for cerebrospinal fluid anti-Hu antibodies and 8 cases were positive. 2. Major clinical manifestations and associated tumors: 15 males and 12 females among the 27 patients; ages 26-76 years. Neurological manifestations included: sensory neuropathy or sensory neuron neuropathy in 9 cases, sensory and motor neuropathy in 5 cases, 2 of which resembled Guillain-Barré syndrome; amyotrophic lateral sclerosis syndrome in 2 cases; subacute cerebellar degeneration in 3 cases (2 of which combined with peripheral neuropathy); brainstem encephalitis in 1 case; encephalopathy caused by inappropriate antidiuretic hormone secretion syndrome in 4 cases; Lambert-Eaton syndrome in 1 case, synovial nerve palsy in 1 case, polymyositis in 1 case, systemic lupus erythematosus (SLE) combined with neuromuscular disease in 1 case, and cerebral infarction in 1 case. Tumor status: 20 cases (74.1%) had tumors including: 17 cases of lung cancer, including 9 cases of small cell lung cancer, 1 case of pulmonary hypofractionated adenocarcinoma and 7 cases of lung cancer of unknown pathological type; 2 cases of gastric cancer; 1 case of abdominal tumor of unknown nature. 8 of the 20 tumor patients were diagnosed with tumor before the appearance of neurological symptoms, 12 cases were diagnosed with tumor after the appearance of neurological symptoms, and 1 of them was diagnosed with lung cancer after 1 year of follow-up. Seven patients were diagnosed with lung cancer after comprehensive examination and no malignant tumor was found. One case was admitted with Guillain-Barré syndrome, which progressed rapidly and died of respiratory failure and gastrointestinal bleeding without autopsy; four cases were followed up for 2 months to 2 years and no malignant tumor occurred; one patient refused comprehensive examination and died of systemic failure after 3 months; one case was lost to follow-up. 3. Positive predictive value of anti-HU antibody: According to the diagnostic criteria of Graus et al. for neurological paraneoplastic syndrome, 22 out of 27 cases could be diagnosed with neurological paraneoplastic syndrome or associated tumors were found, and the 22 diagnosed cases included 2 cases in which no primary tumor was found, which met the diagnostic criteria of Graus et al. for neurological paraneoplastic syndrome, item 1, and the other 20 cases in which tumors were found. The positive predictive value of anti-HU antibodies was 81.5% (22/27). II. Discussion The anti-Hu antibody is a polyclonal IgG antibody whose target antigen is a protein with a molecular weight of 38 kD in the nucleus of neuronal cells, which belongs to RNA-binding proteins and plays an important role in mRNA-encoded proteins . The protein is expressed at a high level in the cerebellar paucino cells and sensory neurons of the posterior root ganglion, making this area more susceptible to anti-Hu antibody action. Some tumor cells, especially small cell lung cancer, also express this protein and therefore induce the production of anti-Hu antibodies with autoantibody properties and mediate autoimmune neurological damage. Anti-Hu antibodies are closely associated with the development of some PNS, which can involve both the central and peripheral nervous systems, and the lesions can be multisystemic or diffuse, or selective, with certain clinical types being relatively common or characteristic, such as subacute cerebellar degeneration, limbic encephalitis, subacute sensory neuronopathy, Lambert-Eaton syndrome, and dermatomyositis. Regarding Hu-positive PNS, Sillevis Smitt P et al. reported 73 cases with clinical types including sensory neuropathy (55%), cerebellar degeneration (22%), limbic encephalitis (15%) and brainstem encephalitis (16%), and 23% with autonomic dysfunction such as gastrointestinal motility disorder ( 14%); tumors were found in 85%, and 77% were lung cancer. Sensory neuron disease and cerebellar degeneration were common in this group, while rare types such as Lambert-Eaton syndrome and amyotrophic lateral sclerosis were also observed. The group also includes 4 cases of SIADH with encephalopathy secondary to severe hyponatremia. Although SIADH belongs to paraneoplastic syndrome, its secondary encephalopathy is not immune-mediated; there is another case of acute cerebral infarction with clinical manifestations not belonging to PNS. Anti-Hu antibody positivity was found incidentally, and although lung cancer was later diagnosed, anti-Hu antibody was considered to be not directly related to cerebral infarction. The anti-Hu antibody test has a high specificity (95%-100%) and sensitivity (more than 80%) for the diagnosis of PNS and its associated tumors. Our study showed that the positive predictive value of anti-Hu antibody was 81.5%, and its diagnostic significance seems to be less than that reported in previous studies. The reasons for this may be twofold: first, we did not follow up some patients long enough to exclude the possibility of tumors yet; second, PNS is a rare disease, but some clinicians use Hu antibodies as a screening test, expanding the indications and scope of the test, which may cause a relative increase in the number of false-positive cases and thus lower the positive predictive value. On the other hand, however, our study is a retrospective analysis of daily clinical tests, reflecting the actual clinical prevalence of the examined population, and therefore our results have major reference significance for domestic neurologists. According to the currently commonly used Graus diagnostic criteria, if the clinical presentation is a classic clinical syndrome, the diagnosis of PNS can be confirmed as long as the anti-Hu antibody is positive, independent of whether a tumor is found. If a tumor has not been found, a comprehensive tumor screening should be performed, and the possibility of lung cancer should be especially alerted; if a tumor is still not found in the comprehensive examination, long-term follow-up and regular review are necessary. In patients with anti-Hu antibody-positive paraneoplastic syndrome, 85% of the potential tumors are small cell lung cancer, and PNS may precede tumors by several years. Considering the high specificity of anti-Hu antibodies for the diagnosis of small cell lung cancer, it is important to be alert to the possibility of combined lung cancer, for example, Auf G et al. reported [7] a patient with anti-Hu antibody-positive PNS with dry syndrome and breast cancer, who was followed up for 3 years and found to have small cell lung cancer. Small cell lung cancer was found after 3 years of follow-up. Therefore, in the present group of two patients with gastric cancer, it remains to be verified whether Hu antibody positivity is attributable to gastric cancer with longer follow-up. Benyahia B et al. reported [9] that out of 173 patients with autoimmune diseases (including 71 cases of dry syndrome and 102 cases of SLE), only one patient with SLE was positive for anti-Hu antibodies, with a positivity rate of 0.6%, and no tumor was found in this case at 5 years of follow-up. antibody positivity can be seen in autoimmune diseases. In fact, the report of Benyahia B et al. precisely illustrates the high specificity of anti-Hu antibodies, because according to their results, the specificity (true negative rate) of Hu antibody positivity for PNS in the differentiation of rheumatic immune diseases such as SLE is 99.4%. This group of cases included one case each of polymyositis and SLE, and although no malignancy was clinically detected, it is necessary to continue follow-up; it is also debatable whether the clinician’s recommendation of anti-Hu antibody testing for these 2 young patients with a relatively clear diagnosis of rheumatic immune disease (26 and 30 years old, respectively) is an expansion of the scope of application of the test. As mentioned earlier, over-indication of the test, or even misuse of laboratory tests may reduce the positive predictive value. In conclusion, anti-Hu antibodies are important for the diagnosis of PNS and related tumors. The classic clinical syndrome combined with a positive Hu antibody can confirm the diagnosis of PNS without necessarily relying on the detection of a tumor. Close follow-up and review is necessary for patients with positive anti-Hu antibodies and no tumor on full examination.