A group of public and private pension funds is suing a number of high profile US exchanges - including NYSE, NASDAQ and Bats - on the grounds that services provided by the exchanges to HFT firms allowed those firms to front-run the pension funds.
Their claim had initially been dismissed by the court on the grounds that the exchanges had immunity. However the pension funds appealed and have found an important ally in the SEC which said immunity applies to the exchanges only when they "act as regulators of their members" and not as service providers.
The case is being heard in the US Court of Appeals for the Second Circuit, which hears appeals for the Court for the Southern District of NY. The latter is the center for much securities market litigation due to its proximity to Wall Street.