Novel optoelectronic devices whose operation is based upon intersubband transitions open new possibilities in the infrared region. Notably, the absorption, gain and refractive index spectra can be tailored to specific applications with a high flexibility dependent almost solely on the design of the structure and not on the actual material parameters. In this paper, to the best of the authors knowledge, the non parabolic electron subbands obtained from the solutions of an 8X8 K.P Hamiltonian are used as input for a many particle optical susceptibility solver capable of consistently taking into account exchange and depolarization effects through numerical inversion of the Nonequilibrium Keldysh Green's Functions equations for the optical polarization. Numerical results are presented for III-V quantum wells for different design, temperature and excitation conditions, and the interplay between many particle and non parabolic band structure effects is discussed. Scattering beyond the exchange approximation and its influence in the lineshape of the transitions is further discussed within a scheme that allows the calculation of increasingly higher order Coulomb correlations under equilibrium and nonequilibrium conditions.