Priming of protective anti-Listeria monocytogenes memory CD8+ T cells requires a functional SecA2 secretion system

Area of Science:

• Microbiology
• Immunology
• Bacterial Pathogenesis

Background:

• The SecA2 secretion system in Gram-positive bacteria, including Mycobacterium tuberculosis and Listeria monocytogenes, exports virulence factors essential for host colonization.
• SecA2 is linked to long-term CD8(+) T cell-mediated protective immunity.
• In L. monocytogenes infections, cytosolic SecA2 correlates with CCL3-secreting memory CD8(+) T cells crucial for secondary infection protection.

Purpose of the Study:

• To investigate whether the SecA2 ATPase itself contains conserved motifs recognized by cytosolic pattern recognition receptors, thereby inducing CCL3(+) memory CD8(+) T cell differentiation.
• To determine if SecA2 or its substrates are responsible for initiating the immune response.

Main Methods:

• Generated a L. monocytogenes mutant with a mutated nucleotide-binding site (NBS) in the SecA2 ATPase.
• Compared the phenotypes of the NBS mutant (colony morphology, protein secretion, virulence) to wild-type and SecA2 deletion mutants.
• Assessed the protective immunity and CD8(+) T cell responses in mice immunized with the SecA2 NBS mutant.

Main Results:

• The SecA2 NBS mutant displayed phenotypes similar to a SecA2 deletion mutant, including impaired secretion and reduced virulence.
• Mice immunized with the SecA2 NBS mutant did not develop protection against secondary L. monocytogenes infection or generate CCL3(+) memory CD8(+) T cells.
• Expression levels of NBS mutant and wild-type SecA2 proteins were comparable, indicating SecA2 itself is unlikely to induce these specific immune cells.

Conclusions:

• The SecA2 ATPase is not directly responsible for inducing CCL3(+) memory CD8(+) T cell differentiation.
• Bacterial substrate proteins secreted via the SecA2 system, rather than SecA2 itself, are likely involved in promoting the generation of protective memory CD8(+) T cells.