RH55 Antibody

Biological Role of RH5 in Malaria Infection

RH5 mediates P. falciparum merozoite invasion of erythrocytes by binding to basigin (CD147) on erythrocyte surfaces . Unlike other malaria antigens, RH5 is essential for parasite survival and exhibits minimal polymorphism, making it a prime vaccine target . Structural studies reveal RH5 adopts a kite-like fold with three-helical bundles, presenting critical epitopes for basigin binding and antibody neutralization .

Key Features of RH5:

• Forms a pentameric complex with CyRPA, RIPR, PTRAMP, and CSS during invasion

• Transiently exposed on merozoite surfaces during erythrocyte engagement

• Antibody binding must occur rapidly to block invasion due to short exposure time

Clinical Vaccine Candidates

Antibody Characteristics

• Natural Infection:

  • RH5-specific B cells are rare (<0.1% of memory B cells)

  • IgG responses are short-lived, with rapid decline post-infection

  • Only 2/186 monoclonal antibodies from infected individuals showed potent neutralization

• Vaccine-Induced Antibodies:

  • Target α-helical core epitopes critical for basigin binding

  • Exhibit faster association rates (kon) correlating with neutralization potency

  • Germline gene combinations (e.g., IGHV3-21/IGLV2-14) linked to high potency

Phase 1b Trial in Tanzanian Infants (NCT03435874)

Notable Outcomes:

• No vaccine-related serious adverse events reported

• Infant sera achieved unprecedented GIA levels (up to 78% at 2.5 mg/mL IgG)

Obstacles in RH5 Vaccination

• Transient antigen exposure limits antibody efficacy

• Non-neutralizing epitopes dominate natural infection responses

• High antibody titers (>300 μg/mL) required for sterilizing immunity

Engineering Solutions

• Stabilized RH5.2: Truncated to remove disordered regions, improving thermostability and immunogen focus

• VLP Conjugation: SpyTag-SpyCatcher bioconjugation to hepatitis B VLPs enhanced immunogenicity 10-fold in mice

• Adjuvant Optimization: Matrix-M outperformed AS01 in eliciting functional antibodies

Future Directions

1. Phase 3 efficacy trials for RH5.1/Matrix-M in Burkina Faso

2. Preclinical testing of multi-epitope vaccines combining RH5 with CyRPA/RIPR

3. Development of bispecific antibodies targeting RH5 and complementary invasion ligands