Evasion of NKG2D-mediated cytotoxic immunity by sarbecoviruses

SARS-CoV-2 and other sarbecoviruses continue to threaten humanity, highlighting the need to characterize common mechanisms of viral immune evasion for pandemic preparedness. Cytotoxic lymphocytes are vital for antiviral immunity and express NKG2D, an activating receptor conserved among mammals that recognizes infection-induced stress ligands (e.g., MIC-A/B). We found that SARS-CoV-2 evades NKG2D recognition by surface downregulation of MIC-A/B via shedding, observed in human lung tissue and COVID-19 patient serum. Systematic testing of SARS-CoV-2 proteins revealed that ORF6, an accessory protein uniquely conserved among sarbecoviruses, was responsible for MIC-A/B downregulation via shedding. Further investigation demonstrated that natural killer (NK) cells efficiently killed SARS-CoV-2-infected cells and limited viral spread. However, inhibition of MIC-A/B shedding with a monoclonal antibody, 7C6, further enhanced NK-cell activity toward SARS-CoV-2-infected cells. Our findings unveil a strategy employed by SARS-CoV-2 to evade cytotoxic immunity, identify the culprit immunevasin shared among sarbecoviruses, and suggest a potential novel antiviral immunotherapy.

The role of OKT3 in clinical transplantation

OKT3 is a murine monoclonal anti-T cell antibody that is directed to CD3, a five-chain molecular complex found in association with the T cell receptor for antigen. OKT3 was the first monoclonal antibody to be used in organ transplantation and during the past 10 years there has been extensive experience of its use both for preventing and treating rejection in organ transplantation. OKT3 blocks T cell function by modulating CD3 and the T cell receptor from the T cell surface. A reaction to OKT3 results from cytokines released when OKT3 first reacts with T cells. This reaction is generally mild but can be severe. First rejections following kidney transplantation are reversed in approximately 95% of cases. Steroid-resistant rejections are also susceptible to OKT3 but in only approximately 75% of cases. When used for prophylaxis, OKT3 completely blocks rejection in 95% of patients and significantly delays the onset of rejection in those who do reject. Antibodies to OKT3 are produced in approximately 75% of patients receiving it. However, seldom are the antibodies to OKT3 present in high titer and only in those cases is successful re-use of OKT3 prevented. As is the case with all potent immunosuppressive drugs, the use of OKT3 is associated with increased viral, specifically cytomegalovirus, infections. However, it appears that reduction of concomitant immunosuppression decreases the incidence of severe infections. Unquestionably, OKT3 has been a useful addition to the immunosuppression used for organ transplantation. In addition, its use has stimulated research on other monoclonal antibodies for use in organ transplantation.

A comparison of 2 protocols for living-related renal transplantation in children: donor-specific transfusions versus cyclosporine

We analyzed the results obtained with protocols for immunosuppression of pediatric recipients of haploidentical living-related renal transplants. In the donor-specific transfusion group transfusion of blood products obtained from the prospective organ donor was performed before transplantation, and at transplantation maintenance immunosuppression of azathioprine and prednisone was begun. In the cyclosporine group donor-specific transfusion was not used, and maintenance immuno-suppression of cyclosporine and azathioprine was begun 1 week before transplantation, with the addition of prednisone at transplantation. Of 24 donor-specific transfusion recipients 3 had circulating cytotoxic antibodies to the prospective donor for an incidence of 12%. There was no significant difference between groups with respect to 1-year actual patient and graft survival (100 and 89 versus 100 and 86%, respectively), 1-year mean serum creatinine level (1.1 versus 1.2 mg./dl.), rejection treatments per patient (2.5 versus 2.6) and total days hospitalized during year 1 after transplantation (27 versus 18), with donor-specific transfusion data presented first. Initial hospitalization was significantly shorter (10 versus 16 days, p less than 0.05) and the incidence of rejection crises within 3 months was significantly less (68 versus 94%, p less than 0.05) in the cyclosporine group. We believe that cyclosporine and azathioprine pre-treatment of pediatric recipients of haploidentical living-related renal transplants with the addition of prednisone at transplantation is preferable to a donor-specific transfusion protocol because there is no risk of recipient sensitization to the prospective donor, and patient and graft survival is not adversely affected.

Monoclonal antibody OKT3 therapy in pediatric kidney transplant recipients

Thirty-one pediatric patients with acute renal allograft rejection were treated with the monoclonal antibody OKT3. In 24 cases, increased doses of steroids followed by a polyclonal antithymocyte globulin were ineffective in reversing the rejection episode. Twenty-eight patients completed the prescribed minimum 10-day treatment course, with effective rejection reversal in 22. Three patients failed to complete the course of therapy: one because of leukopenia that developed after the first dose, one because of a clotted graft, and another because of symptomatic cytomegalovirus infection. The overall success rate of OKT3 for rejection reversal was 74%; however, 55% of recipients had rebound rejection, and 85% of patients had detectable anti-OKT3 antibodies after completion of the course of therapy. Ten patients were treated with a second course of OKT3, and in eight of these patients, rejection was at least temporarily reversed. The starting dose of OKT3 for second-course therapy was the same as that used during first-course therapy, but in five cases the dose was increased during the course because of inadequate therapeutic response. Seven of these patients lost their grafts a mean of 6.5 months after completion of second-course therapy. We looked for anti-OKT3 antibody in nine recipients after completion of a second treatment course and found it in all nine. Our observations regarding a second treatment course with this monoclonal antibody preparation suggest that although rejection reversal may be observed, ultimate graft survival is poor and anti-OKT3 antibody formation is enhanced.

OKT3 monoclonal antibody in pediatric kidney transplant recipients with recurrent and resistant allograft rejection

Twelve pediatric patients, aged 28 months to 17 years, received OKT3 to reverse renal allograft rejection. In 11 patients, the rejection crisis was resistant to conventional antirejection therapy with high doses of prednisone or polyclonal antithymocyte globulin. Reversal of rejection was successful in 10 patients who completed a treatment course. Because of recurring resistant rejection, five patients received a second course of OKT3, which was successful in reversing the rejection crisis in two. Among these patients, the persistence or the appearance of high levels of circulating T3 lymphocytes after initiating the second treatment course correlated with treatment failure. The immediate side effects associated with OKT3 therapy were transient and medically manageable. We conclude that this monoclonal antibody preparation is a safe and effective treatment for pediatric renal allograft in recipients experiencing rejection crisis resistant to conventional therapy. However, the potential impact of this immunosuppressive medication on long-term renal allograft survival in this patient population remains to be determined.

Early experience with continuous arteriovenous hemofiltration in critically ill pediatric patients

The applicability of continuous arteriovenous hemofiltration (CAVH) for renal replacement therapy was evaluated in three infants and two young children with catastrophic medical and surgical illnesses. In the first four patients, CAVH was used in conjunction with either peritoneal or hemodialysis. In the fifth patient, CAVH was the sole renal replacement therapy employed; in this critically ill anuric infant, we were best able to evaluate the ability of CAVH to continuously control fluid, electrolyte, and acid-base balance, and allow the administration of adequate parenteral nutrition. The difficulties encountered were related to anticoagulation, establishment of adequate vascular access, and selection of an appropriate hemofilter for the performance of the technique. Despite the application of suction-assistance, we were unable to effectively employ a prototype pediatric hemofilter to attain a level of plasma ultrafiltration consistent with the objectives of therapy. However, we were able to effectively and safely employ an adult hemofilter for these purposes; modifications were made in the adult hemofilter system before its application in the smallest pediatric patients. Our experience suggests that, even in critically ill infants, CAVH can be successfully applied as an effective renal replacement therapy. However, further experience is required before its potential impact on patient survival can be assessed.