In my last presentation, I talked about the article by King et al. describing a method for computationally removing T-cell receptor epitopes from proteins. The work could have significant impact on the field of designing protein therapeutics, where immunogenicity is a serious obstacle.
One of the major challenges when developing a protein therapeutic is the activation of the immune system by the drug and subsequent production of antibodies against it, rendering the therapeutic ineffective. This process is known as immunogenicity. Immunogenicity is triggered by T-cells recognition of peptide epitopes displayed on the MHC (major histocompatibility complex). This recognition can be impeded by designing the protein therapeutic to remove the potential T-cell epitopes from its surface. There has been some success in experimental T-cell epitope removal, but the process remains resource and time consuming.
In this work, King et al. created a function which assigns to each residue a score that measures its propensity to be a part of a T-cell epitope. The score consists of three parts. The first part is based on a SVM (Support Vector Machine) score calculated over each 15-residue long window, that attempts to predict how likely is the corresponding peptide sequence to bind the MHC. The SVM has been trained on the available immunological data from the Immune Epitope Database (IEDB). The second part of the score is calculated on each 9-residue window and compares the frequency of the 9-mer in the host genomic data and in the known epitope data (a sequence occurring with a high frequency in a human genome would be rewarded while the opposite is true for sequences occurring in the known epitope data). The third part penalizes any deviations from the original charge of the protein. These three parts are combined with a standard Rosetta score that measures the stability of the protein. The weights assigned to each segment were calibrated on existing protein structures. The combined score would be used to score the mutations in the sequence of the protein of interest, according to their propensity of reducing immunogenicity. The top scoring mutations would then be combined in a greedy fashion.
The authors tested their method on fluorescent reporter protein superfolder GFP (sfGFP) and the toxin domain of the cancer therapeutic HA22. In the case of sfGFP the authors targeted the four top-scoring T-cell epitopes. They created eight different proteins designs, out of which all preserved the function of the original protein (fluorescence). The authors selected the top scoring design for experimental immunogenicity testing. The experiments have shown that the selected design had a significantly reduced immunogenicity in comparison to the original protein. In the case study of HA22 the authors created five designs, out of which three displayed cytotoxicities at the same level or higher than the original protein. The two most cytotoxic designs were further characterized experimentally for their propensity to induce immune response. The authors have found that the two mutants elicited a significantly reduced T-cell response.
Figure 1: Reduction of immunogenicity without loss of function. A) Three of the five designs show cytotoxicity at the same level or higher than the original protein. B) Two of the three cytotoxicity-preserving designs show reduced immunogenicity
Overall, this very interesting study showed that computational methods can be successfully used for reducing immunogenicity of protein therapeutics, opening new avenues for computational protein design.
