TLR agonists and antitumor vaccine efficacy

Paper reading-Journal for Immunotherapy of cancer

Introduction

The combination of toll-like receptor (TLR) agonists with immune checkpoint blockade (ICB) has been explored as a strategy to enhance the efficacy of cancer vaccines. This research investigates how combining TLR3 and TLR9 agonists with different immune checkpoint inhibitors affects the antitumor immune response in murine tumor models.

The study primarily focuses on the interaction between these agents and their impact on regulatory T cells (Tregs) and CD8+ T cells within the tumor microenvironment.

Methods

1. Animal Models and Cell Lines:

• Mice: C57BL/6 mice were used for all experiments.
• Cell lines: E.G7-OVA (ovalbumin-expressing) and MyC-CaP (prostate cancer) tumor cell lines were used.

2. Peptides and TLR Agonists:

• Peptide: SIINFEKL (OVA peptide) used for immunization.
• TLR Agonists: TLR3 agonist (Poly(I)) and TLR9 agonist (ODN 1826) were coadministered with vaccines.

3. In Vitro Assays:

• OT-II splenocyte stimulation and Treg suppression assays were conducted to evaluate immune responses.

4. Tumor Treatment Studies:

• Antibody treatments included αPD-1, αCTLA-4, αLAG-3, αTIM-3, αVISTA, and IgG controls.
• Tumors were assessed for growth, and immune cell populations were analyzed by flow cytometry.

Results

1. Combining TLR Agonists with αCTLA-4:

• Improved the efficacy of peptide-activated OT-I CD8+ T cells in suppressing E.G7 tumor growth.
• The addition of TLR agonists further suppressed tumor growth compared to TLR agonists or αCTLA-4 alone.

2. Impact on Tregs:

• TLR agonists suppressed PD-1 expression on Tregs and activated these cells, leading to enhanced suppressive function.
• Depletion of Tregs in tumor-bearing mice improved the antitumor efficacy of the combination therapy, even with αPD-1 present.

3. Combination with αLAG-3:

• Peptide-activated CD8+ T cells had greater antitumor efficacy against E.G7 tumors when delivered with TLR agonists and αLAG-3 compared to αPD-1 and αLAG-3.
• This combination led to increased expression of 4–1BB on tumor-infiltrating CD8+ T cells.

4. Combination Therapy in Prostate Tumor Model:

• The combination of TLR agonists and αCTLA-4 or αLAG-3 significantly suppressed tumor growth and prolonged survival in the MyC-CaP model.
• Increased numbers of tumor-infiltrating CD8+ T cells and a higher CD8-to-Treg ratio were observed.

Discussion

The study demonstrates that TLR agonists, particularly TLR3 and TLR9, when combined with certain immune checkpoint inhibitors (such as αCTLA-4 or αLAG-3), can significantly enhance the efficacy of cancer vaccines. However, the combination with αPD-1 showed less efficacy due to the activation of Tregs. The results suggest that optimal combinations of TLR agonists and immune checkpoint blockade need to be carefully selected to improve the efficacy of anticancer vaccines.

Critical Points

1. TLR Agonists as Vaccine Adjuvants:

• TLR agonists can modulate the tumor microenvironment and enhance the antitumor immune response by decreasing PD-1 expression on CD8+ T cells and increasing CTLA-4 expression.

2. Role of Tregs:

• The activation and suppression of Tregs play a critical role in determining the efficacy of combination therapies. Depletion of Tregs may be necessary when using TLR agonists with αPD-1 to improve vaccine efficacy.

3. Immune Checkpoint Blockade Specificity:

• Different immune checkpoint inhibitors have varying effects on T cell activation and tumor suppression. The study highlights the importance of selecting the appropriate checkpoint inhibitors for combination with TLR agonists.

Future Directions:

• Further research is needed to explore other TLR agonists and their combinations with various immune checkpoint inhibitors in different tumor models. Evaluating other functional aspects of CD8+ T cell activation will also be crucial.

Conclusion

This study underscores the potential of combining TLR agonists with immune checkpoint blockade to enhance the efficacy of cancer vaccines. By understanding the interactions between these agents and the tumor microenvironment, more effective immunotherapeutic strategies can be developed to improve outcomes for cancer patients.

References

1. Jeon D, Hill E, Moseman JE, McNeel DG. Combining toll-like receptor agonists with immune checkpoint blockade affects antitumor vaccine efficacy. J Immunother Cancer. 2024;12. doi:10.1136/jitc-2024–008799.