CD40LG and pneumococcal infection: In order to further examine the mechanism for the high susceptibility of CD22-/- mice to pneumococcal infection, secreted IgM and GM-CSF was measured in the bronchoalveolar lavage (BAL) of infected mice, because natural IgM that is produced by GM-CSF secreting B1a–like cells has been shown to be crucial for protective anti-bacterial responses in the lung [13].