As we all know, when bacteria, viruses, and other microorganisms invade the human body, the body removes harmful microorganisms through Innate immunity and adaptive immunity. It produces immunoglobulins that can bind to target antigens through adaptive immunity, which can accurately and efficiently remove harmful substances. However, some COVID-19 patients can still clear the virus without producing specific antibodies. What are the characteristics of these patients, how to rely on Innate immunity to remove the virus, what is the relationship between novel coronavirus and human immune response? In this study, we analyzed the clinical characteristics and clinical outcomes of COVID-19 patients without serum conversion using electronic medical records. We reported there was no significant difference in gender, age, non-survivors and the clinical classification between the antibody-negative and antibody-positive groups. Fever, dyspnea, fatigue, expectoration, and myalgia, which were observed more in the antibody-positive group. In terms of laboratory findings, white blood cell counts (WBC), Neutrophils, C-reactive protein, Procalcitonin, Interleukin-6 (IL-6), Lactate dehydrogenase, Creatine kinase, etc, were significantly higher in antibody-negative patients than in antibody-positive group. Specially, 143 (81.2%), 20 (18.3%), 34 (44.2%) antibody-negative blood samples having increased C-reactive protein, Procalcitonin, Interleukin-6 and 37 (19.9%) antibody-negative blood samples having Lymphocytopenia. Our results showed that innate immunity played an essential role in clearing the virus.
The common symptoms of the antibody-negative patients included in this study were cough, fever, dyspnea, fatigue expectoration, and myalgia. The incidence rate was lower than that of the antibody-positive group and other related studies[4, 5]. This might be related to the inhibition of virus activity and weakening of virus pathogenicity by an inflammatory reaction in vivo. The types of coexisting illness and the proportion of severe/critical cases in the antibody-negative group were similar to those in the antibody-positive group and other studies[6, 7], suggesting that the antibody non-responders accorded with the overall distribution characteristics of COVID-19 patients. Wang, B., et al. studied the relationship between antibody and virus clearance time in 26 patients with COVID-19; they found the early production of antibodies does not mean early elimination of this virus, not observe a correlation between early adaptive immune responses and better clinical outcomes. One of the cases did not produce specific antibodies to SARS-CoV2 within 66 days of observation. However, eventually, the nucleic acid test of SARS-CoV2 turned negative, revealing that some individuals may not produce antibodies after being infected with SARS-CoV2. In this study, it was found that 94 of the 1921 patients did not produce specific antibodies. However, the virus nucleic acid was still cleared during hospitalization, which confirmed that the production of specific antibodies by adaptive immunity was not necessary for the clearance of SARS-CoV-2.
C-reactive protein (CRP) is a non-specific inflammatory marker and a kind of acute-phase reaction protein, which can activate complements, strengthen phagocytes' function, and remove pathogenic microorganisms invading the body and tissue cells that are damaged, necrotic and apoptotic. Interleukin-6 is also a non-specific indicator of inflammation. Inflammatory cytokines produced by various cells after inflammatory stimulation are the critical components of the inflammatory response and can induce the increase of CRP and PCT at 2h and 6h after infection, respectively. PCT reflects the activity of systemic inflammation, which increases slightly when the virus is infected. When the pathogen invaded the body, the human immune system entered the immediate innate immune response stage, neutrophils were the central effector cells, and the total number of leukocytes and neutrophils increased significantly. In the early stage of the innate immune response, activated neutrophils produce pro-inflammatory cytokines such as interleukin-6, while hepatocytes produce a series of acute-phase proteins after being stimulated by pro-inflammatory cytokines such as interleukin-1, of which C-reactive protein is the most significant. Zhu, L, et, al found that interferon-stimulated genes (such as ISG15, IFI44L, and MX1) in peripheral blood immune cells of patients with COVID-19 were significantly up-regulated, which confirmed that the innate immune response was significantly activated in patients with COVID-19.
Meanwhile, it was found that the concentration of interleukin-6 (IL-6) in patients with COVID-19 was significantly higher than that in ordinary people. We did not find that the total number of white blood cells, neutrophils, C-reactive protein, IL-6, PCT, and other inflammatory cells and factors were higher in the antibody-negative group in this study. It was confirmed that after SARS-COV-2 entered the body, the innate immune response of this population was rapid, intense, and cleared the virus quickly. Other studies found that pathogenic T cells were activated rapidly to produce GM-CSF and IL-6, and then GM-CSF further activated CD14+, CD16+ inflammatory monocytes to produce more IL-6 and other inflammatory cytokines, resulting in an inflammatory storm. The number of lymphocytes-decreased patients in the antibody-negative group was more than that in the antibody-positive group, suggesting that the inhibition of the virus on lymphocytes weakened the adaptive Immune response and delayed the production of specific neutralizing antibodies by B lymphocytes. Strong inflammatory reactions spread throughout the body, involving many target organs such as the liver and kidney, resulting in a significant increase in lactate dehydrogenase, creatine kinase, creatine kinase isoenzyme, urea nitrogen, and creatinine.
The virus clearance time of antibody-negative patients was significantly shorter than that of antibody-positive groups. The shedding time of viruses in other related studies varied greatly from 11 to 20 days[12, 13]. After eliminating the confounding factors such as age and coexisting illness, the hospitalization time of the antibody-negative group was significantly shorter than that of the antibody-positive group, which confirmed that the rapid clearance of the virus was related to the short hospitalization time. 3/94(3.2%) deaths were all critically ill patients, and the mortality rate was lower than the current results of related studies[5, 14]. the activation of innate immune response is necessary to eliminate the invading virus effectively, but its abnormal activation and excessive production of pro-inflammatory cytokines may cause damage to the host tissue.Galloway, J. B., et al. scored COVID-19 patients on 12 items, and the higher the score, the greater the risk of CCU or death, including neutrophil count>8.0 x10 9 /L, CRP>40 mg/L. Another study listed leukocyte > 10x109/L and neutrophil > 7.5x109/L as risk factors for adverse outcomes. In another paper, it was found that SARS-CoV-2 infected human macrophages could induce the release of intracellular ISG15 to extracellular through papain-like protease PLpro encoded by SARS-CoV-2, and extracellular free ISG15 continued to amplify the expression of a variety of pro-inflammatory cytokines and chemokines in a manner similar to cytokines, which is one of the possible causes of excessive inflammatory response in patients with COVID-19.Whether these studies suggest that excessive inflammation is related to the death of critically ill patients is worthy of further study.
Our study has some notable limitations. First, because of the retrospective study, there was a lack of more detailed laboratory testing of immune cell inflammatory factors, such as all kinds of T cell count, tumor necrosis factor, and various types of interleukins and so on, to more accurately judge the degree of the inflammatory response of COVID-19 patients. Second, there was a significant gap between the antibody-negative group and the positive group. Although We used the 1:1 propensity score matching; the bias has inevitably affected our assessment.