Suspension of thymic emigration promotes the maintenance of antigen-specific memory T cells and the recall responses

• Timely decline in thymic output benefits memory T cells.
• Thymic involution is important for protection against secondary infection.
• Thymic involution may have a positive physiological significance.

Thymic involution is evolutionarily conserved and occurs early in life. However, the physiological significance remains elusive of this seemingly detrimental process. The present study investigated the potential impact of altered thymic output on T cell memory using ovalbumin (OVA) expressed by Listeria monocytogenes as a model antigen. Suspension of thymic emigration by thymectomy was shown to lead to a marked increase in the frequency and total number of OVA-specific memory T cells. In contrast, oversupply of thymic emigrants through thymic grafting caused a significant decrease of such cells. When rechallenged with L. monocytogenes expressing OVA, the thymectomized mice mounted a more potent recall response as evidenced by the enlarged population of OVA-specific tetramer+ cells and the accelerated clearance of the bacteria. Notably, the memory-enhancing effect of thymectomy was abrogated following weekly adoptive transfer of naive T cells. Together, data from the present study indicate that reduced thymic output favors the maintenance of the memory T cell pool.