SARS-CoV-2, a novel coronavirus that originated in Wuhan, China, has produced a global pandemic known as COVID-19 with substantial mortality and morbidity 1 . To infect the host, SARS-CoV-2 uses the viral receptors ACE2 and TMPRSS2, which are membrane associated proteins expressed in many cells throughout the body, particularly the respiratory system 2 . Knowledge about the immunopathology of COVID-19 is evolving, and there is heterogeneity of symptoms among the affected population. Most cases are mild, but in a number of patients, the disease evolves into an acute respiratory distress syndrome (ARDS) 3 or a dysregulated immune system state leading to cytokine storm, most often in older adults, requiring intensive care and resulting in increased mortality 4 . There is growing evidence that some younger patients may have worse outcomes as well, which may be related to host factors, including an altered immune system, genetics, and comorbidities like obesity and smoking 1 . Based on these observations, there is concern that patients treated with immunosuppressive drugs may be at a higher risk of developing poor outcomes during a SARS-CoV-2 infection. Therefore, there is great interest in determining the impact of COVID-19 in patients on disease-modifying therapies (DMTs) for autoimmune neurological diseases like multiple sclerosis (MS), neuromyelitis Optica spectrum disorder (NMOSD) and others. An important concern is how B cell depletion modifies the host ability to mount an effective response to SARS-CoV-2. Here, we describe an AQPR4+ NMOSD patient treated with rituximab who developed mild COVID-19 infection despite B cell depletion. We also discuss the emergent evidence of the host cellular and humoral immunity against SARS-CoV-2.
This report was considered exempt research by the UConn Health IRB. A 59-year old woman presented with transverse myelitis in 2006 and was initially diagnosed with multiple sclerosis (MS). She had subsequent relapses with optic neuritis in each eye separately and once simultaneously. MRI of the spine and brain in 2017 showed a left-sided long segment cord signal abnormality extending from C2 to C6 with associated mild cord atrophy, stable since 1 year prior, stable short segment myelitis of the thoracic cord at the level of T2-T3 without evidence of restricted diffusion or abnormal enhancement, and multiple small foci of T2/FLAIR signal hyperintensity within the cerebral white matter bilaterally. She was recategorized in 2014 as NMOSD, AQPR4-positive based on white matter disease in the brain, longitudinal transverse myelitis in the cervical spine, and positive serology for AQPR4 antibody (Fig. 1A, B) . She was initially started on azathioprine, then transitioned to rituximab in early 2014 resulting in chronic B cell depletion and stable disease. In early February 2020 (day 1), the patient developed chills and fever of 102.4 F and low 100s for four days, with no cough, pharyngitis, dyspnea. She went to her PCP on day 5. Rapid influenza A and B tests were negative, other labs were normal, and she was diagnosed with a "fever of unspecified cause." Fever, cough, and malaise persisted, and in early March 2020 (day 26), she presented to Neurology for a routine follow-up clinic visit.
She reported continued flu-like symptoms and malaise. Temperature was 99.5 ºF and O2 saturation was 96%. She denied international travel to areas of high incidence of COVID-19 or contact with infected individuals. Tamiflu was prescribed. Despite concern for COVID-19, she could not get SARS-CoV-2 PCR testing because of limited availability. A chest x-ray was normal.
She was instructed to self-quarantine and monitor for worsening respiratory status. On day 28, she developed sinus congestion with postnasal drip. She was afebrile. Her PCP diagnosed acute viral sinusitis and told her to continue Tamiflu for a total of 5 days. A week later, she had worsening fever of 103.5 ºF, rigors, headache, myalgias, and mild dyspnea. She presented to the emergency department, where she was tachycardic with a temperature of 100.4 ºF, and O2 saturation was Multiple Sclerosis and Related Disorders, 2020 94% on room air. Tests for influenza, adenovirus, enterovirus, parainfluenza virus types 1-4, rhinovirus, human metapneumovirus, and RSV were negative. SARS-CoV-2 RNA PCR was positive, and she was admitted. During her hospital stay, she developed mild headaches and chest tightness but no dyspnea. Chest x-ray showed only mild prominence of the bronchovascular markings (Fig. 1C) . CD19/CD20 quantification by flow cytometry indicated depletion of B cell (Fig.   1D ). Absolute neutrophil count (ANC) was elevated and was associated with a decreased of the Absolute lymphocyte count (ALC) at 500 per L at the time of her most severe symptoms and fever (Fig. 1E) . She did not require mechanical ventilation and responded to supportive therapy and was discharged home after 3 days.
Our patient had a prolonged flu-like illness that was either a prolonged course of COVID-19 or more likely a different viral respiratory infection followed by SARS-CoV-2. We cannot distinguish between these two possibilities since she was not tested for SARS-CoV-2 early in her presentation. During the COVID19-positive phase of her illness, she had only mild respiratory symptoms with moderate constitutional symptoms. She did not develop overt respiratory insufficiency and changes in the chest X-ray were mild, with minimal lab abnormalities. She had severe lymphopenia that recovered (Fig 1E) , indeed the reappearance of CD8+ T cells precede the resolution of symptoms and is consistent with an effective immune response to SARS-CoV-2 in non-severe cases. 5 Lymphopenia is seen frequently, and recent work suggests that sustained lymphopenia, especially CD8+ T cells is an independent predictor for COVID-19 severity 6 Her neutrophil population (ANC) increased at the time of increased disease severity, and then declined as her lymphocyte population partially recovered (Fig 1C) . The ratio of ANC/ALC has been suggested as a readily available marker of disease severity in COVID infection 7 .
There is a suggestion that in some patients the severe disease is due to immune dysregulation, in particular, cytokine storm, therefore targeting cytokines such as IL6 can be Multiple Sclerosis and Related Disorders, 2020 beneficial 4 . This suggestion is supported by reports of cases of mild disease in a patient with long term corticosteroid use 8 but there is a need for prospective randomized trials to determine if immunosuppression or immunomodulation may be protective from worse outcome of COVID19.
The impact of Immunosuppressive medications on the course of COVID-19, especially DMTs like rituximab that deplete circulating B cells, is of particular concern. There are reports of severe and even fatal viral infections in patients on rituximab. These include encephalitis due to Coxsackie A16 9 , Enterovirus 10 11 , Powassan 12 , Tick-Borne Encephalitis 13 , West Nile 14 , JCV neurological disease including PML 15 16 and granule cells neuronopathy 17 . Our patient did not present any new neurological signs or symptoms and there was no indication for a lumbar puncture. Currently, it is unclear if SARS-CoV-2 is neurotropic. The SARS-CoV-2 receptors ACE and TMPRSS2 are not highly expressed in the brain, although preliminary evidence from single cell RNA sequencing data suggest that the olfactory epithelium expresses these receptors. This may explain the cases of anosmia seen in COVID-19. There is preliminary evidence that oligodendrocytes may express COVID receptors, and recently there was a case of encephalitis with SARS-CoV-2 found in the CSF 18 . Moreover, neurotropism of coronaviruses may be independent of the expression of viral receptor in brain cells 19 
A) Brain MRI shows lack of typical periventricular T2 lesions but shows sparse scattered T2 hyperintensities in the deep white matter and corpus callosum, atypical for MS (arrows). B) Cervical spine MRI demonstrates of longitudinal extensive T2 hyperintensities from C2-C6 (arrows). C) Anteroposterior chest radiographs during impatient hospitalization that showed alteration in lung fields with increasing markings but not consolidation. D) FACS data demonstrating absence of CD20 population in circulating peripheral blood mononuclear cells due to rituximab E) Quantification of ANC, ALC and ANC/ALC ratio during the symptomatic phase of the infection. Arrows indicate points of maximal decline on ALC that correspondent to increase in ANC and vice versa.