As shown in Figure 2A, cathepsin D, the most abundant cathepsin in differentiated macrophages [31], underwent proteolytic maturation in phagolysosomes following pneumococcal infection, as evidenced by detection of the heavy chain form of active cathepsin D. We also confirmed that the organelles isolated on a sucrose gradient were phagolysosomes by identifying markers of phagolysosomes such as LAMP-1, rab-5 and -7 (Figure S2). Here, CTSD is linked to pneumococcal infection.