To this end gravity flow columns with a cut-off value of 6000 Da (Econo-Pac 10DG) were used according to the minimal dilution protocol instructions supplied by themanufacturer (Bio-Rad Laboratories SA/NV, Eke, Belgium). In brief, 3 ml of sterile supernatant was poured onto the column and a first fraction, containing the higher molecular weight
components (N6000 Da), was eluted with 4 ml MilliQ water.