The Human Immunodeficiency Virus (HIV) envelope protein, known as Env, is the sole viral component protruding from the surface of the virus particle. Env is responsible for recognizing and infecting human immune cells, making it the primary target for vaccine development. The native Env protein is initially produced as a precursor called gp160. Researchers engineered a soluble version called gp140, which has become a focus for creating an effective HIV vaccine. This engineered protein is designed to mimic the natural structure of the virus’s outer spike, providing a stable immunogen for the immune system to recognize and produce neutralizing antibodies.
Molecular Identity and Structure
The native HIV envelope spike begins as the gp160 precursor protein, which is later cleaved by a cellular enzyme into two non-covalently associated subunits: gp120 and gp41. The outer subunit, gp120, is responsible for binding to the host cell receptors. The inner subunit, gp41, is the transmembrane component that anchors the complex to the viral surface. The mature, functional spike on the virus surface is a trimer, composed of three copies of the gp120/gp41 heterodimer complex.
gp140 is a laboratory-engineered construct created by truncating the native gp160 protein, specifically removing the transmembrane and cytoplasmic domains of gp41. This modification results in a soluble protein that is secreted from the cell rather than being anchored to a membrane. Structurally, gp140 is defined as the ectodomain of the Env protein, encompassing the entire gp120 subunit and the external portion of the gp41 subunit. It is designed to stabilize three copies of this ectodomain into a trimer, mimicking the overall shape of the functional spike.
gp140 is often stabilized using specific mutations to prevent it from falling apart, a common issue with the native Env protein. For instance, the SOSIP trimer incorporates an engineered disulfide bond (SOS) between gp120 and gp41 and a proline mutation (IP) in gp41. This locks the complex into a native-like, prefusion conformation. Creating a stabilized trimer is necessary because this structure is the form the immune system must recognize to generate protective antibodies.
Function in Viral Entry
The native Env trimer, which gp140 mimics, initiates HIV infection by mediating the virus’s entry into host cells. This process relies on the sequential action of the gp120 and gp41 subunits. The initial step involves the gp120 subunit binding to the CD4 receptor, predominantly found on helper T-cells.
Binding to the CD4 receptor triggers a conformational change in gp120, exposing a binding site for a secondary co-receptor, such as CCR5 or CXCR4. This two-step binding stabilizes the virus’s attachment to the target cell membrane. The resulting conformational changes are then transmitted to the gp41 subunit, which executes the final step of entry.
The gp41 subunit undergoes structural rearrangement, extending a fusion peptide that inserts into the host cell membrane. The protein then folds back on itself, forming the stable six-helix bundle, which acts like a molecular winch. This action pulls the viral envelope and the host cell membrane together, causing them to fuse and creating a pore. Through this pore, the viral genetic material enters the cell cytoplasm, completing the infection process.
Role in HIV Vaccine Development
The goal of using gp140 in vaccine development is to induce broadly neutralizing antibodies (bNAbs) that can recognize and disable the functional Env spike on diverse HIV strains. Unlike earlier efforts using the monomeric gp120 subunit, gp140 presents the target epitopes in their native, trimeric quaternary arrangement. This stabilized, native-like structure is necessary to elicit specific antibodies that target the conserved, hidden regions of the Env protein.
The native Env protein is structurally complex and highly shielded by a dense layer of sugar molecules, or glycans, that mask vulnerable sites. Furthermore, Env is highly variable across different HIV strains, meaning antibodies generated against one strain often fail to neutralize others. Vaccine strategies using engineered gp140 trimers attempt to overcome these hurdles by preferentially exposing conserved epitopes that are less prone to mutation.
The gp140 trimer guides the immune system to produce antibodies that bind to sites required for viral function, such as the CD4 binding site or the membrane-proximal external region of gp41. Studies show that gp140 trimers are superior to gp120 monomers at inducing neutralizing antibodies in animal models. However, the response is not yet sufficient to protect against the full diversity of circulating HIV strains. Current research explores ways to further stabilize the gp140 trimer and display it on delivery platforms to enhance the immune response.
Manufacturing and Delivery Techniques
The production of gp140 relies on established recombinant protein technology. The gene sequence encoding gp140 is inserted into a vector and introduced into host cells for expression. Mammalian cell lines, such as HEK293 cells, are frequently used because they correctly perform the complex folding and glycosylation necessary to resemble the native viral spike.
Once host cells have produced and secreted the soluble gp140 protein, it must be purified from the culture medium using techniques like affinity chromatography and size exclusion chromatography. This process isolates the correctly folded trimeric form of the protein from misfolded or monomeric byproducts. Alternative production systems, including transient expression in plants, offer potential for rapid scalability and reduced infrastructure requirements.
The purified gp140 protein is often formulated with adjuvants, which are substances designed to boost the immune response to the antigen. Adjuvants are used to stimulate immune cells, ensuring the body generates a robust antibody response against the gp140 immunogen. Delivery systems also include displaying gp140 on the surface of self-assembling nanoparticles, such as ferritin cages or virus-like particles, to enhance immunogenicity.