Development of T cells begins in the thymus where thymocytes, during their maturation stage rearrange their T cell receptor (TCR) repertoire. A positive and a negative selection take place, which ensures the generation of a competent T cell pool able to recognize foreign antigens. The goal of the positive selection is that only those T cells survive, which express TCRs recognizing self (or host) MHC molecules. In a healthy organism alloreactive T cells undergo a negative selection by which T cells reacting with self-peptides are eliminated. However, some T cells bearing autoreactive TCR can become regulatory T cells after interacting with a single self-peptide with high affinity (thymic Tregs or t Tregs) [4]. These immunoregulatory CD4+CD8−CD25+ thymocytes can control self-reactive T cells