Macropinocytosis is an evolutionarily-conserved, large-scale, fluid-phase form 
of endocytosis that has been ascribed different functions including antigen 
presentation in macrophages and dendritic cells, regulation of receptor density 
in neurons, and regulation of tumor growth under nutrient-limiting conditions. 
However, whether macropinocytosis regulates the expansion of non-transformed 
mammalian cells is unknown. Here we show that primary mouse and human T cells 
engage in macropinocytosis that increases in magnitude upon T cell activation to 
support T cell growth even under amino acid (AA) replete conditions. 
Mechanistically, macropinocytosis in T cells provides access of extracellular AA 
to an endolysosomal compartment to sustain activation of the mechanistic target 
of rapamycin complex 1 (mTORC1) that promotes T cell growth. Our results thus 
implicate a function of macropinocytosis in mammalian cell growth beyond 
Ras-transformed tumor cells via sustained mTORC1 activation.