Computational Methods for Electron Tomography of Influenza Virus

Abstract

Influenza is a rapidly changing virus that appears seasonally in the human population. Every year a new strain of the influenza virus appears with the potential to cause a serious global pandemic. Knowledge of the structure and density of the surface proteins is of critical importance in a vaccine candidate. Reconstruction techniques from a series of tilted electron-tomographic projection images provide quantification of surface proteins. Two major categories of reconstruction techniques are transform methods such as weighted backprojection (WBP) and series expansion methods such as the algebraic reconstruction techniques (ART) and the simultaneous iterative reconstruction technique (SIRT). Series expansion methods aim at estimating the object to be reconstructed by a linear combination of some fixed basis functions and they typically estimate the coefficients in such an expansion by an iterative algorithm. The choice of the set of basis functions greatly influences the result of a series expansion method. It has been demonstrated repeatedly that using spherically symmetric basis functions (blobs), instead of the more traditional voxels, results in reconstructions of superior quality, provided that the free parameters that occur in the definition of the family of blobs are appropriately tuned. In this chapter, it is demonstrated that, with the recommended data-processing steps performed on the projection images prior to reconstruction, series expansion methods such as ART (with its free parameters appropriately tuned) will provide 3D reconstructions of viruses from tomographic tilt series that allow reliable quantification of the surface proteins and that the same is not achieved using WBP.