Detection of SARS-CoV-2 antibodies in pediatric kidney transplant patients

The coronavirus disease 2019 (COVID-19) pandemic is considered a public health emergency. Despite all efforts and policies to contain it, severe acute respiratory syndrome (SARS-CoV-2) infection continued to spread rapidly. As of October 8, 2020, COVID-19 has led to over 1 million deaths and 3.5 million cases worldwide [1] Approximately 85% of cases will be mild, while 10% will have moderate disease requiring hospitalization, and 5% will have severe disease requiring ICU admission [2]

As pediatric solid organ transplant recipients are considered a high-risk group and are prone to more severe viral, bacterial, and fungal infections, this pandemic has raised concerns among the transplant community about managing this vulnerable group. In the early days of the pandemic, COVID-19 was thought to be more severe among immunocompromised patients. The fear of more severe disease has limited kidney transplant surgeries to those that are urgent and has led to the closure of many living transplant programs [3], including ours. This perceived risk in transplant patients is an extrapolation of data from other viruses. Unlike influenza and adenovirus [4], SARS-CoV-2 does not seem to preferentially affect immunocompromised patients. There is emerging evidence that immunosuppressed children exhibit mild disease [5‐7]. This mild disease course could be explained by the role of immunosuppressants in damping of the dysfunctional or hyperimmune response that occurs in the later stages of the infection, leading to lung tissue injury and acute respiratory distress syndrome (ARDS) [8]. Although we are one year into the pandemic, little is known about the effect of COVID-19 on the pediatric kidney transplant (PKT) population.

To better understand the SARS-CoV-2 infection prevalence, clinical presentation and the rate of seroconversion in the PKT population, we aimed to perform universal SARS-CoV-2 serological testing in all our pediatric kidney transplant patients attending outpatient clinics.

Most SARS-CoV-2 infections in children with intact immune responses are mild. However, severe infection can rarely occur [14]. The difference in disease severity between adult and pediatric populations might be related to a more robust innate immune response and higher levels of interleukin-17 and interferon gamma in children [15],, which play an important role in early disease containment. This difference in disease severity could also be explained by the fact that children are more prone to recurrent viral infection that can cause epigenetic changes in their trained immune system, making it more efficient at fighting the virus [16]. Immunocompromised patients are always at a higher risk of contracting severe viral and bacterial infections. Understanding the host immune response of the kidney transplant population to SARS-CoV-2 infection is evolving but remains unclear.

Herein, we reported our experience in using serological testing as a screening tool for COVID-19 PKT recipients. Of the 72 PKT recipients who underwent serological testing, the prevalence of COVID-19 based on two positive serological tests for SARS-COV-2 IgG was 11.1% (8 of 72). The patient with positive PCR and negative seroconversion was found to have profound B cell lymphopenia a few months before her infection, which could explain the lack of antibody production in this case. Few case reports from multiple sclerosis patients who were on B lymphocyte-depleting agents showed either negative SARS-CoV-2 seroconversion or an attenuated antibody response [17, 18]. However, no one knows how these patients would respond to vaccination. Another explanation for the negative seroconversion in this patient could be that she lost her antibodies to SARS-CoV-2 by the time she was tested.

It is well known that the early adaptive humoral immune response involving the development of neutralizing antiviral antibodies is an important mechanism in eliminating viral replication and providing protection from reinfection [19]. Our patients demonstrated high antibody titers against spike proteins tested by Diasorin assay, which was recently found to correlate well with neutralizing antibodies [20]. These findings may indicate that pediatric kidney transplant patients are capable of producing protective immunity post infection. In our series, all patients with positive serology, including the one with positive PCR results without seroconversion, were either asymptomatic or had mild symptoms not requiring hospital admission. None of them developed pneumonia or required oxygen supplementation. Our findings are consistent with what has been reported so far. A study done by Marlais et al., on 113 COVID-19 immunocompromised children with kidney disease, including 53 transplant patients, found that most of the children with kidney disease taking immunosuppressive medications have mild SARS-CoV-2 infection [21]. Another large multinational observational study performed by the Italian Society of Pediatric Nephrology included 1572 children with CKD or on immunosuppressants, of whom 288 (18%) were kidney transplant patients, none of them contracted severe SARS-CoV-2 infection during the study period [22].

Furthermore, a large multicenter seroprevalence study including 992 children of health care workers living in the UK found that 68 (6.8%) were positive for SARS-CoV-2 antibodies, 50% of them were asymptomatic, and the rest had only mild symptoms [23]. In another study that enrolled 13 patients with inflammatory bowel disease on immune suppressants who were diagnosed with COVID-19 based on a positive PCR, seroconversion occurred in 4 out of the 6 tested patients (66.7%), one patient had an equivocal serological test, and one did not exhibit seroconversion. Notably, 46.2% of the patients were completely asymptomatic despite no alteration in their baseline immunosuppressants; 53.8% were mildly symptomatic, and fever was the most common symptom [24].

After reviewing the limited available literature about SARS-COV-2 infection in the pediatric transplant population, we found that transplantation and an immunocompromised state do not increase the risk of ICU admission or mortality in the pediatric population. However, data in the adult transplant population showed contradictory results. In the pediatric transplant population, there is some evidence from multiple case series showing that immunosuppressant medications might have a beneficial effect. Bush et al. reported a case of a pediatric kidney transplant recipient who had a mild course of the disease despite being on immunosuppressant medications. In a series of 200 pediatric liver transplant patients in a large transplant center in Italy, 3 cases of COVID-19 were reported, and none of these patients had severe pneumonia or a severe course [25]. This observation led the author to conclude that immunosuppressant medications might have a protective role.

In a systematic review of 16 articles including children and adults with cancer and kidney, heart, liver and immunodeficiency patients, immunocompromised patients had similar and sometimes more favorable outcomes than the general population. However, this favorable outcome was not observed in cancer patients [26].

In our study, serological testing identified five patients with COVID-19 who otherwise would have been missed because of mild or asymptomatic disease. Although none of those five patients had severe symptoms or complications that required hospital attention, identifying them helped us better understand the full clinical spectrum of the disease in this vulnerable group. We learned that asymptomatic COVID-19 is not uncommon in the PKT population.

Out of the four patients with documented positive PCR, 75% seroconverted with relatively high antibody titers. Two patients had readings above the detectable upper limits of the test (> 400 AU/ml). Although these levels are very high, no one truly knew how these levels translate into protection from the virus. These findings were unexpected, as we already know that compared to the regular population, transplant patients who developed infection have a blunted immune response and a low rate of seroconversion [27].. The fact that our transplant patients mounted a strong humoral immune response to natural infection makes us believe that they will elicit the same, if not stronger, immune response with future vaccination when it becomes available for this age group. More interestingly, all patients who mounted rigorous immune responses to COVID-19 were still positive at a median time of 75 days (IQR, 69–83 days). One patient (case 2) was still positive for SARS-CoV-2 IgG at 120 days. The fact that all our positive serology patients had a history of contact with SARS-CoV-2 PCR-positive household members validates the positive serology results in our cohort. It also supports the hypothesis of possible household transmission among children that was addressed by Buonsenos et al [28]. The one patient (case 9) who had only one positive test was consider as negative. Of note, the parents’ antibody titers from approximately the same time as those assessed their children were significantly lower than those in their children except in one patient. These findings are in contrast to what has been reported by Pierce et al., who showed that serum neutralizing SARS-CoV-2 IgG antibodies were higher in adults than in children [15].

Importantly, no major modifications were made to the immunosuppressant medication in any PCR-positive patients except for one patient (case 3) in whom the MMF dose was reduced by half for one week. All patients but one developed a humoral immune response to COVID-19. This high rate of seroconversion in the PKT population has been replicated in adults and a few pediatric studies. In a recently published work by Prendecki et al., they found a relatively high seroprevalence rate of 10.4% in a cohort of adult kidney transplant recipients [29] In another small cohort of the PKT population that included 31 patients, seroconversion was reported in only one patient who was completely asymptomatic, and 3 patients had intermediate positive results [30]. Two adult kidney transplant patients reported seroconversion between days 19 and 23 after the reduction of immunosuppressants [31].. There was also documented seroconversion in another two adult kidney transplant patients who developed SARS-CoV-2 antibodies at 30 days post infection [32]. Moreover, a study performed by Hartzell et al. reported 100% seroconversion in 16 SARS-CoV-2-positive adult kidney transplant patients [33]. Furthermore, Choi et al. also reported a 100% seroconversion rate in 5 adult KTR patients [34]. Some of the current adult transplant guidelines including the guidelines from the Canadian society of transplantation pediatric group [35] suggest reduction or complete withdrawal of immunosuppressants. However, this suggestion was challenged by multiple studies that showed the ability of CNI to inhibit in vitro viral replication of SARS-CoV-2 infection independent of its immunosuppressant effects [36]. Furthermore, cyclosporine was successfully used to treat cases of secondary hemophagocytic lymphohistocytosis (HLH) that overlaps with the cytokine release storm, which seems to be the leading cause of mortality in COVID-19 patients [37].

To the best of our knowledge, our study is the first to report a relatively high seroprevalence of COVID-19 in the PKT population. This is the first study to test the humoral immune response to SARS-CoV-2 infection using a quantitative serological assay that correlates with neutralizing antibodies in this population. However, similar to all pediatric kidney transplant studies, we were limited by a small sample size, especially in this highly protected group. Moreover, the results cannot be generalized to all PKT patients, as the majority of the patients enrolled were more than one year post transplantation and were on low-dose maintenance immunosuppressants. The immune assays used were IgG based, so acute infection could have been missed. Importantly, all samples obtained were from Saudi children who might have different immunological backgrounds that can affect the immunoassay performance characteristics. For all these mentioned reasons, further larger studies with wider geographical samples and longer durations of antibody titer follow-up are needed.

The seroprevalence of SARS-CoV-2 infection in our study is the highest reported among the PKT population so far. Screening PKT patients using serological testing was effective in identifying the whole spectrum of the disease, including asymptomatic patients. On the other hand, symptom-based screening for SARS-CoV-2 infection might not be an effective tool in identifying who should be tested, as asymptomatic infection is not uncommon and can lead to disease spread in hospital settings and in public. Most importantly, the humoral immune response to SARS-CoV-2 infection was rigorous and persistent in the PKT population, which will have positive implications for future vaccination. Immunosuppressants seem to not exert an additional risk for a more severe course in this population, and a reduction in immunosuppressants might not be necessary.