A powerful mechanism of vertebrate innate immunity has been discovered in the past year, in which APOBEC proteins inhibit retroviruses by deaminating cytosine residues in nascent retroviral cDNA. To thwart this cellular defence, HIV encodes Vif, a small protein that mediates APOBEC degradation. Therefore, the balance between APOBECs and Vif might be a crucial determinant of the outcome of retroviral infection. Vertebrates have up to 11 different APOBEC proteins, with primates having the most. APOBEC proteins include AID, a probable DNA mutator that is responsible for immunoglobulin-gene diversification, and APOBEC1, an RNA editor with antiretroviral activities. This APOBEC abundance might help to tip the balance in favour of cellular defences.

APOBEC3G (CEM15 ) deaminates cytosine to uracil in nascent retroviral cDNA. The potency of this cellular defense is evidenced by a dramatic reduction in viral infectivity and the occurrence of high frequencies of retroviral genomic-strand G --> A transition mutations. The overwhelming dinucleotide hypermutation preference of APOBEC3G acting upon a variety of model retroviral substrates is 5'-GG --> -AG. However, a distinct 5'-GA --> -AA bias, which is difficult to attribute to APOBEC3G alone, prevails in HIV-1 sequences derived from infected individuals (e.g., ). Here, we show that APOBEC3F is also a potent retroviral restrictor but that its activity, unlike that of APOBEC3G, is partially resistant to HIV-1 Vif and results in a clear 5'-GA --> -AA retroviral hypermutation preference. This bias is also apparent in a bacterial mutation assay, suggesting that it is an intrinsic APOBEC3F property. Moreover, APOBEC3F and APOBEC3G appear to be coordinately expressed in a wide range of human tissues and are independently able to inhibit retroviral infection. Thus, APOBEC3F and APOBEC3G are likely to function alongside one another in the provision of an innate immune defense, with APOBEC3F functioning as the major contributor to HIV-1 hypermutation in vivo.

The human proteins APOBEC3F and APOBEC3G restrict retroviral infection by deaminating cytosine residues in the first cDNA strand of a replicating virus. These proteins have two putative deaminase domains, and it is unclear whether one or both catalyze deamination, unlike their homologs, AID and APOBEC1, which are well characterized single domain deaminases. Here, we show that only the C-terminal cytosine deaminase domain of APOBEC3F and -3G governs retroviral hypermutation.

Potent neutralising monoclonal antibodies are one of the few agents currently available to treat COVID-19. SARS-CoV-2 variants of concern (VOC) that carry multiple mutations in the viral spike protein can exhibit neutralisation resistance, potentially impacting the effectiveness of some antibody-based therapeutics. Here, generation of a diverse panel of 91 human neutralising monoclonal antibodies provides an in-depth structural and phenotypic definition of receptor binding domain (RBD) antigenic sites on the viral spike. These RBD antibodies ameliorate SARS-CoV-2 infection in mice and hamster models in a dose-dependent manner and in proportion to in vitro neutralising potency. Assessing the impact of mutations in the spike protein on antibody recognition and neutralisation highlights both potent single antibodies and stereotypic classes of antibodies that are unaffected by currently circulating VOC such as B.1.351 and P.1. These neutralizing monoclonal antibodies, and others that bind analogous epitopes, represent potentially useful future anti-SARS-CoV-2 therapeutics.

Reslizumab and mepolizumab are recently approved monoclonal antibodies for the treatment of severe (uncontrolled) eosinophilic asthma. Both are effective in neutralizing the function of interleukin-5 (IL-5). This study is the first to compare the binding affinity and in vitro potency of both antibodies in head-to-head assays. Two assays assessed binding affinity (using the equilibrium dissociation constant [K D ]) of each drug for human IL-5. In the Biacore surface plasmon resonance assay, the association constant (k on ) values for human IL-5 for reslizumab and mepolizumab were 3.93 × 10 6 and 1.83 × 10 5 , respectively. The dissociation constant (k off ) values were 4.29 × 10 −4 and 2.14 × 10 −4 , respectively. Calculated K D values for human IL-5 for reslizumab and mepolizumab were 109 and 1,170 pM, respectively, representing an approximately 11-fold stronger binding affinity with reslizumab. In the Kinetic Exclusion Assay, the k on values for human IL-5 for reslizumab and mepolizumab were 3.17 × 10 6 and 1.32 × 10 5 , respectively. The k off values were 1.36 × 10 −5 and 1.48 × 10 −5 , respectively. Measured K D values for human IL-5 for reslizumab and mepolizumab were 4.3 and 112 pM, respectively, representing an approximately 26-fold stronger binding affinity for reslizumab. A human-IL-5-dependent cell proliferation assay was developed to assess in vitro potency, based on a human cell line selected for enhanced surface expression of IL-5 receptor-alpha and consistent proliferation response to IL-5. The concentration at which 50% inhibition occurred (IC 50 ) was determined for both antibodies. Reslizumab and mepolizumab inhibited IL-5-dependent cell proliferation, with IC 50 values of approximately 91.1 and 286.5 pM, respectively, representing on average 3.1-fold higher potency with reslizumab. In conclusion, comparative assays show that reslizumab has higher affinity binding for and in vitro potency against human IL-5 compared with mepolizumab. However, these results do not take into consideration the different methods of administration of reslizumab and mepolizumab.