The crystal structure of BTLA/HVEM complex (A and B chains) served as a starting point for the modeling (Protein Data Bank ID 2AW2) . The atoms of the HVEM B chain were removed, except for residues 23–39. The protonation state of the histidines was set depending on the environment, to optimize hydrogen bonds. All simulations were carried out using GROMACS 4.5 molecular modeling package  using the CHARMM22 force field . The structure was located in the center of a TIP3P water box in which the distance between the solute and the edges was 1.4 nm. Na+ and Cl- ions were added to neutralize the system. Subsequently the system was subjected to 1000 steps of steepest descent minimization. The structures were then equilibrated according to standard equilibration procedures. The system heating was performed using simulated annealing method by increasing the temperature in four steps: from 0K to 50K, then from 50K to 100K, from100K to 200K and finally from 200K to 300K. Every step took 100ps and at the end of the procedure the system was additionally coupled to the target 300K temperature for 100ps. The force constant of 1000kJ/mol*nm was applied to keep the atoms in their positions during the heating procedure. The same force constant was kept also during the first constant volume equilibration simulation during 100ps, followed by another 100ps simulation in the NVT scheme with lowered force constant for position restraint to 300kJ/mol*nm. The final step of equilibration consisted in a simulation in the NPT scheme with Nose-Hoover thermostat and Parinello-Rahman barostat with 300K and 1Pa for 500ps and no position restraints imposed on atoms.