Disclosed herein are methods for treating an immune system condition and/or altering T cell activation in a subject. Also disclosed herein are methods for diagnosing an immune system condition in a subject. In some examples, the methods can include measuring expression of at least one T cell activation-related miRNA in a sample obtained from a subject. The methods further include administering to the subject a therapeutically effective amount of an miRNA or mimic thereof, and/or an inhibitor of miRNA or mimic thereof and/or administering to the subject T cells contacted with an effective amount of miRNA or mimic thereof, and/or an inhibitor of miRNA or mimic thereof.