We study the BPS spectrum of Little String Theory for bound states of M5branes wrapped on six manifold of product topology M 4 ×Σ 2 and the apparence of multi-loop θ-functions in a supersymmetric index calculation. We find a total reconstruction of the gloop heterotic contribution in the case of a double K3 M-theory compactification. Moreover, we consider total wrapping of M5-branes on del Pezzo surfaces B k and, by studying the relevant amplitude, we notice the arising of θ-functions relative to BPS strings on T k−1 , i.e. membranes on T k . This happens because of beautiful relations between four dimensional SYM theories and CFTs in two dimensions and seems to be linked to a duality recently observed by A.Iqbal, A.Neitzke and C.Vafa in [1]. 1