CD28 and influenza: Expression of CD28 on Tfh in the medLN and Th1 cells in the lung was determined 12 days following influenza infection: 30–40% of Tfh cells (Figure 2H) and 55–65% of lung Th1 cells (Figure 2I) had lost CD28 expression in Cd28flox/floxOx40cre/+ mice.