New Concept of First Quantization
  We derive a Lagrangian density of Dirac field by employing the local gauge invariance and the Maxwell equation as the fundamental principle. The only assumption made here is that the fermion field should have four components. The present derivation of the Dirac equation does not involve the first quantization, and therefore this study may present an alternative way of understanding the quantization procedure. 
T. Fujita, S. Kanemaki and S. Oshima