CD4 and Mycoplasma pneumoniae pneumonia: To confirm the SIgA responses promoted by CTSL contribute to the resistance to mycoplasma pneumonia, pigs injected with rCTSL before they were challenged with M. hp, showed milder clinical symptoms and histopathological damage of lungs, less mycoplasma burden together with higher secretion of SIgA, percentages of CD4+ T cells and level of MHC II molecules comparing with the group without rCTSL.