In addition to causing cancer, bacteria may also mark cancer cells to help the immune system recognize them

Earlier this year, an article talked about the relationship between bacteria and cancer, and that some bacteria can directly cause cancer. Recent studies have found that things are far from that simple. Peptides produced by bacteria that invade tumor cells can be presented on the surface of tumor cells and recognized by the human immune system. These bacterial peptides can be used as a kind of "small flag" inserted into cells as tumor antigens that have never been recognized, sending out signals to promote cancer immune recognition and treatment.

Tumor antigens are targets recognized by the immune system and are mainly divided into two categories: tumor-associated antigens and tumor-specific antigens.

Tumor-associated antigens are expressed in both normal tissues and tumors, so it is not easy to activate immune responses. Tumor-specific antigens are only expressed on tumor cells, so they are ideal targets for specific immune attacks on tumors.

There are a variety of bacteria that can invade human tumors and modulate their immune function, ultimately affecting cancer patient survival and their response to treatment.

The researchers investigated the bacterial composition of cancer cells from patients whose melanoma had metastasized and found that the bacterial composition of different metastases from the same person was highly similar, and sometimes even samples from different people, indicating that specific bacteria are common in melanoma, which is consistent with the results of a study of tumor microbiota of different types of cancer presented in January. The researchers also confirmed that these bacteria are present in melanoma cells, but not in the surrounding extracellular microenvironment.

The researchers found nearly 300 peptides in 33 bacterial samples, several of which appeared repeatedly in multiple tumors of the same person and in tumors of different people. They may be "small flags" used as markers.

For the human body, bacterial peptides are "non-self" and tend to trigger strong immune responses more easily without worrying about autoimmunity. Therefore, the bacterial peptides identified in this study may become important potential targets for immunotherapy, which is a rare and useful therapeutic combination.

Perhaps in the near future, bacteria will become a “good helper” for humans in the fight against cancer!

Kalaora, S., Nagler, A., Nejman, D. et al. Identification of bacteria-derived HLA-bound peptides in melanoma. Nature (2021).

doi.org/10.1038/s41586-021-03368-8