* Futures up: Dow 22 pts, S&P 2.25 pts, Nasdaq 8.25 pts
By Tanya Agrawal
June 22 (Reuters) - U.S. stock index futures were slightly higher on Wednesday as investors held off from making big bets, a day ahead of Britain's referendum on its European Union membership.
* Traders are guardedly optimistic about a "Remain" vote. While polls are extremely close, bookmakers have pointed to betting patterns in favor of the U.K. remaining in the EU.
* Investors will also keep an eye on U.S. Federal Reserve Chair Janet Yellen's second day of testimony to Congress on the health of the economy.
* On Tuesday, Yellen was optimistic about the economy and played down the risk of a recession, but warned that the British vote on Thursday and a U.S. hiring slowdown posed risks to the economic outlook.
* Yellen's comments pointed to a cautious central bank, virtually ruling out a July rate hike. The Fed also warned U.S. stock market valuations are "well above" their median over the past 30 years, the strongest such assessment in years.
* The S&P 500 is trading at about 16.5 times expected earnings, above the 30-year median of 14.6 times, according to Thomson Reuters Datastream.
* Data on Wednesday includes U.S. home resales at 10 a.m. ET (1400 GMT). A Reuters survey of economists forecast a 1.1 percent rise in existing home sales to an annual rate of 5.54 million units last month, following a 1.7 percent gain in April.
* Oil rose above $50 a barrel after an industry report showed a large drop in U.S. crude inventories, with analysts expecting volatile trading ahead of the British referendum.
* Tesla Motors was down 11.7 percent at $193.90 in premarket trading after the Elon Musk-owned electric car maker made an offer to buy his solar installation firm SolarCity in a stock deal worth as much as $2.8 billion. SolarCity was up 16.6 percent at $24.70.
* Adobe Systems was down 4.6 percent at $95.10 after its second-quarter revenue and full-year revenue forecast just about met analysts' estimates.
* Winnebago Industries jumped 11 percent to $23.99 after the motorhome maker's quarterly results beat expectations.
Futures snapshot at 7:12 a.m. ET:
* S&P 500 e-minis were up 2.25 points, or 0.11 percent, with 111,854 contracts traded.
* Nasdaq 100 e-minis were up 8.25 points, or 0.19 percent, on volume of 15,139 contracts.
* Dow e-minis were up 22 points, or 0.12 percent, with 15,233 contracts changing hands. (Reporting by Tanya Agrawal in Bangalore; Editing by Don Sebastian)