These pumps, usually restricted to intracellular acidic organelles,31 are expressed on the surface of certain cell types including tumor cells where they were proposed to be a target of PPIs.32 Interestingly, CD14+ monocytes, unlike CD3+ lymphocytes, are positive for surface v-ATPases and positivity increases following LPS stimulation (Figures 4b and c), suggesting a possible role of surface-bound v-ATPases as PPI receptors on activated monocytes. Here, CD14 is linked to neoplasm.