(Reuters) -Wall Street's main indexes closed lower on Monday, driven by a drop in AI leader Nvidia that weighed on tech stocks, as investors looked ahead to a crucial inflation report set for later this week. Nvidia fell 2.5% after China's market regulator launched an investigation into the chipmaker over suspected violation of antimonopoly law, dragging the information technology sector down 0.45%. Advanced Micro Devices slipped 5.7% after BofA Global Research downgraded its rating on the stock, weighing on the Philadelphia Semiconductor Index, which slipped 0.87%.