For the past decade, biologists have tried to address that issue by exposing various human cells to Tat and documenting their responses. It's an understatement to say that there's a lack of consensus about the results of those efforts. How Tat functions inside the cell is now fairly well understood. There are enough fairly credible publications that one has to take seriously the concept that Tat has effects outside the cell. What those may be is a morass in the scientific literature.
Many factors contribute to the confusion over Tat.
For one, the protein isn't easy to keep in its biologically active state. Another complication may result from there being two forms of Tat, one with 72 amino acids and one with 101. The two proteins can trigger profoundly different responses, says Eric M. Verdin of the University of California, San Francisco. In fact, some of his experiments suggest opposing roles for the two Tats. The long version seems to act as an immune suppressor, whereas the short one stimulates immune cells.