*4.1. The Potentially Protective E*ff*ect of Immunosuppression Relating to COVID-19 Stage III, a Hypothesis Based on Preliminary Observations in SOT*

With respect to the COVID-19 case fatality in patients with chronic immunosuppression after SOT, both a higher incidence of disease and mortality could be expected. Surprisingly, this has not been the case so far. Until mid April, only seven documented cases in patients with SOT had been reported, whereas SARS-CoV-2 had by then resulted in more than 2,000,000 infections worldwide. In Italy, patients with COVID-19 (irrespective of clinical stage) required ICU admission in 12% of the total SARS-CoV-2 positive cases presenting with any kind of symptom and sampled for virus material, and 16% of all hospitalized patients [39]. In China, case fatality was 49.0% in critical cases (1023 of 2087), and in Italian patients it has been reported to be high as 7.2%, although final numbers from the ongoing coronavirus crisis are still pending [39].

One possible explanation for these unexpectedly low numbers could be that immunosuppression in SOT patients protects against the dramatic elevation of pro-inflammatory cells in the presence of COVID-19. It possibly mitigates the hyperinflammation ("cytokine storm") that can be observed in immunocompetent patients with COVID-19 Stage III. There is some evidence that this is true for the calcineurin inhibitor tacrolimus (see below). Secondly, a cytopathic effect due to the virus could play an important role. Viruses can kill the human cells in which they reproduce, leading to cellular damage in the infected organs. The question of whether immunosuppressive therapy also can mitigate the viral cytopathic effect remains unanswered. Reports on autopsy in COVID-19 patients are extremely rare. Tian et al. described the pathologic findings in two COVID-19 patients, showing edema and prominent proteinaceous exudates, vascular congestion, and inflammatory clusters with fibrinoid material and multinucleated giant cells. Reactive alveolar epithelial hyperplasia and fibroblastic proliferation (fibroblast plugs) is indicative of early organization [40].

The mechanism of a possible beneficial effect of immunosuppressive therapy remains unclear. One paradigm could be the effect of immunosuppressive therapy on both the innate and adaptive immunity to SARS-CoV-2. The innate immune response includes cells such as IL-1, -2, -3, -6, TNF-α and IFN-γ, trying to protect the human cells from infection and to eliminate the virus, occurring well before the adaptive immunity becomes activated. The adaptive immunity has two major divisions, which are the antiviral B-cell (antibody mediated) and T-cell immune response. The antibody-mediated response binds to free viral particles, in order to block infection of the host cells. This part of the immune response, however, has more importance in preventing reinfection, which is currently the focus in developing vaccines against SARS-CoV-2. In contrast, the T-cell division of the adaptive immunity is much more important for resolution of the virus than the B-cell response. T-cells are needed for recognizing and destroying SARS-CoV-2 infected cells and the coordination of the whole machinery of the inflammatory response. An overshoot of this inflammatory response could lead to organ damage (cytokine storm, hyperinflammation).

In most patients with SOT, the maintenance immunosuppression includes calcineurin inhibitors (CNIs, namely tacrolimus or cyclosporine), an antiproliferative agent (mycophenolate mofetil (MMF) or azathioprine) and low-dose corticosteroids (prednisone or prednisolone) as maintenance therapy.

The CNIs impair upregulation of (among others) interleukin (IL)-2, thereby reducing the proliferation, maturation and survival of T-cells, impairing an effective immune response. They also inhibit IL-4, TNF-α and IFN-γ. Corticosteroids also reduce the expression of many molecules that are needed in the immune response, such as IL-1, -2, -3, -6, TNF-α and IFN-γ. The antiproliferative agents diminish the clonal expansion of the alloreactive T-cells.

In this way, a cytokine storm could possibly be prevented in SOT patients. Therefore, immunosuppression is probably not a risk factor, but rather beneficial in this population, although the number of observations is very low, not allowing definitive conclusions yet. Even in patients with a lung transplantation, who generally have a more profound immunosuppressive therapy compared to patients with other SOT, a higher risk of incidence and severity of COVID-19 has not been observed so far. Moreover, in the described cases, none of the patients had signs of a major acute or chronic allograft dysfunction, another known complication of respiratory viral infections, particularly in lung transplant recipients. On the other hand, by blocking the above-mentioned important components of the antiviral innate immune response, one would expect the incidence (not severity) of (mild) COVID-19 to be increased, or at least be equal to that among immunocompetent individuals. On the contrary, in the medical literature there is a surprisingly low number of case reports on SOT patients with COVID-19. This could be an under-reported group of patients, or the low number may be related to the fact that these patients have been aware of their susceptibility to infections since being transplanted, and thus act more prudently in the context of the pandemic than the non-transplanted population for whom these measures are largely new and not yet routine behavior. However, more studies concerning these questions and a longer follow-up are needed to draw more firm conclusions concerning these aspects of the pandemic.

What do we learn from studies in other coronaviruses? As seen in MERS, there is a potential role for tacrolimus [41]. One case report described two renal transplant recipients who tested positive for MERS CoV. The patient under tacrolimus had a full recovery, whereas the other patient, who was not on this treatment, did not survive the infection [42]. In vitro, in studies of the pathways of the viral replication of coronavirus, tacrolimus effectively inhibited the viral replication of SARS-CoV, coronavirus NL63 and 229E [43]. This was confirmed with a tacrolimus derivative in another laboratory study [44]. Although these studies are not specific to COVID-19, evaluation of tacrolimus could be interesting in the treatment of COVID-19.

Mycophenolate mofetil as a potential therapy for MERS and SARS-CoV has also been studied. Although in laboratory studies it seemed to inhibit both MERS-CoV and SARS-CoV, in an animal experiment with marmosets it showed high viral loads with more severe or even fatal disease [45–47].

The role of corticosteroids in SARS-CoV is not conclusive. They were widely used during the SARS-CoV outbreak, but can promote viral rebound and acute respiratory distress syndrome [48]. Importantly, in animal experiments with dexamethasone, it was suggested that in pigs with SARS-CoV infection, dexamethasone could reduce the early pro-inflammatory response, but a prolonged administration could promote viral replication [49]. In a human study that separated SARS-CoV patients into four treatment groups, the best response was seen in the group receiving early high-dose corticosteroids [50].
