Fc-mediated functions of nirsevimab complement direct respiratory syncytial virus neutralization but are not required for optimal prophylactic protection

Abstract

Introduction: Nirsevimab is an extended half-life (M252Y/S254T/T256E [YTE]-modified) monoclonal antibody to the pre-fusion conformation of the respiratory syncytial virus (RSV) Fusion protein, with established efficacy in preventing RSV-associated lower respiratory tract infection in infants for the duration of a typical RSV season. Previous studies suggest that nirsevimab confers protection via direct virus neutralization. Here we use preclinical models to explore whether fragment crystallizable (Fc)-mediated effector functions contribute to nirsevimab-mediated protection.

Methods: Nirsevimab, MEDI8897* (i.e., nirsevimab without the YTE modification), and MEDI8897*-TM (i.e., MEDI8897* without Fc effector functions) binding to Fc γ receptors (FcγRs) was evaluated using surface plasmon resonance. Antibody-dependent neutrophil phagocytosis (ADNP), antibody-dependent cellular phagocytosis (ADCP), antibody-dependent complement deposition (ADCD), and antibody-dependent cellular cytotoxicity (ADCC) were assessed through in vitro and ex vivo serological analyses. A cotton rat challenge study was performed with MEDI8897* and MEDI8897*-TM to explore whether Fc effector functions contribute to protection from RSV.

Results: Nirsevimab and MEDI8897* exhibited binding to a range of FcγRs, with expected reductions in FcγR binding affinities observed for MEDI8897*-TM. Nirsevimab exhibited in vitro ADNP, ADCP, ADCD, and ADCC activity above background levels, and similar ADNP, ADCP, and ADCD activity to palivizumab. Nirsevimab administration increased ex vivo ADNP, ADCP, and ADCD activity in participant serum from the MELODY study (NCT03979313). However, ADCC levels remained similar between nirsevimab and placebo. MEDI8897* and MEDI8897*-TM exhibited similar dose-dependent reduction in lung and nasal turbinate RSV titers in the cotton rat model.

Conclusion: Nirsevimab possesses Fc effector activity comparable with the current standard of care, palivizumab. However, despite possessing the capacity for Fc effector activity, data from RSV challenge experiments illustrate that nirsevimab-mediated protection is primarily dependent on direct virus neutralization.

Keywords: Fc-mediated effector function; RSV immunoprophylaxis; anti-RSV F protein monoclonal antibodies; nirsevimab; respiratory syncytial virus.

Figure 1

Evaluation of nirsevimab and palivizumab Fc effector activity in vitro. Nirsevimab, palivizumab, and R347 (range: 2.29–5000 ng/mL) were evaluated for in vitro (A) ADNP, (B) ADCP, (C) ADCD, and (D) ADCC activity. Samples were tested in technical triplicates with the (A, B) average phagocytic score, (C) average complement deposition (measured as median florescence intensity), or (D) relative luminescence units (RLUs) shown for each mAb concentration. An AUC analysis of each mAb tested across eight dilutions is shown for each respective assay. The mean AUC value (± SD) is reported with an ordinary one-way ANOVA with post-hoc Dunnett multiple comparisons test. Not significant (ns); p>0.05; *p<0.05; **p<0.01.

Figure 2

Evaluation of ex vivo nirsevimab Fc effector activity in participant serum from the Phase 3 MELODY study. Ex vivo nirsevimab (A) ADNP^†, (B) ADCP, (C) ADCD, and (D) ADCC activity was measured using participant serum from the Phase 3 placebo-controlled MELODY study (NCT03979313). Nirsevimab-immunized and placebo participant serum pools were diluted in PBS. Diluted pools were tested in technical triplicates at the dilutions shown (range: 10^–6–10^–2) with the (A, B) average phagocytic score, (C) average complement deposition (measured as median florescence intensity), or (D) relative luminescence units (RLUs) for each dilution shown. An AUC analysis for all dilutions within each serum pool is shown for each respective assay. The mean AUC value (± SD) is reported with a linear mixed-effect model of log (AUC) and false discovery rate correction to compare Fc effector activity between nirsevimab-immunized serum on Day 1 and 15; nirsevimab Day 15 and placebo Day 15; and nirsevimab Day 15 and IgG-depleted serum. Three pools of nirsevimab samples were tested at Baseline and at Day 15. For comparison with placebo time points and IgG-depleted serum, nirsevimab sample results were combined into one pool for each time point. The reported SD reflects the technical replicates of the three separate pools. ^†One replicate of IgG-depleted serum was excluded from the ADNP analysis, as its phagocytic score ranged from 1120–2037, which exceeded the highest scores seen in all the other Fc effector activity groups. Not significant (ns); p>0.05; *p<0.05; ***p<0.001; ****p<0.0001.

Figure 3

Evaluation of nirsevimab-mediated protection from RSV infection in cotton rats. (A) Weight-based doses of MEDI8897*, MEDI8897*-TM, or R347 negative control mAb were administered to cotton rats 1 day prior to challenge with RSV A2 (1 x 10⁶ plaque-forming units [PFU] per rat). Animals were sacrificed 4 days after challenge and RSV titers in (B) lungs and (C) nasal turbinate were determined via plaque assay. Data shown are mean RSV PFU per gram of tissue (pfu/g) and SD for 6–8 animals per group. Significance was evaluated by one-way ANOVA. LLOD, lower limit of detection; ULOD, upper limit of detection; ns not significant.