Barclays losing its share of the American Airlines credit card portfolio to Citi is a negative development, potentially impacting its credit card business negatively.

The announcement of a new exclusive credit card partnership with Citi is a significant positive development for American Airlines, leading to a 15% increase in its stock price. This deal likely enhances AAL's financial prospects and market position.

Citi's new exclusive partnership with American Airlines is a strategic win, potentially increasing its credit card portfolio and customer base, which is positive for its business outlook.