Immunogenicity of snake α-neurotoxins and the CD4 T cell epitopes

Snakebite envenomation is an important medical problem in numerous parts of the world causing about 2.7 million envenomations and between 81,000 and 138,000 deaths ayear. Antivenoms (AVs) are time proven effective therapeutics for snakebite envenomation. However, AVs, especially those against elapid neurotoxic venoms (cobras, kraits and mambas), are difficult to produce and are generally of low neutralizing potency. The most lethal component of most elapid venoms is the postsynaptic neurotoxins or the α-neurotoxins, which are responsible for death in most victims. It is generally believed that the low neutralizing potency of the AVs is due to the small molecular sizes, and thus the low immunogenicity, of the α-neurotoxins. Therefore, modifications of the toxins have been made to increase their size, and/or to detoxify them, hoping to improve the toxin's immunogenicity and AV potency. However, these maneuvers have not been applied to commercial AV production. The α-neurotoxins belong to a group of small proteins called three-finger toxins (3FTxs). The 3FTxs contain about 60-77 amino acid residues with four to five disulfide linkages and three anti-parallel β-sheets, which extend from a globular hydrophobic core resembling three fingers. The members of the 3FTxs exhibit a number of important pharmacological activities, e.g., inhibition of neuromuscular transmission and acetyl cholinesterase activities. Recent immunization experiments with a 26 amino acid peptide containing the consensus sequence of the α-neurotoxins, and a mixture of elapid α-neurotoxins using highly effective adjuvants and immunization protocols have resulted in neutralizing antibodies in rabbit and horse, respectively. In the present report using bioinformatics, we show that 23 3FTxs which include α-neurotoxins, cardiotoxins and non-conventional toxins, and the 26 amino acid peptide, were all predicted to contain high to medium score CD4 T-cell epitopes for human and mouse MHC IIs. This information corroborates the results obtained from animal experiments that the α-neurotoxins, in spite of their small sizes and toxicity, are in fact immunogenic. Thus, the uses of effective adjuvants and immunization procedures, rather than chemical/physical modifications of the toxin structures, are crucial to the production of potent AVs against elapid neurotoxic venoms.