Scientists create viral vector to boost immunity to cancer

A modified virus neutralizes the protein responsible for the immunosuppressive activity of lymphocytes, facilitating attacks on tumor cells.

By Karina Toledo, in Caxambu (MG)

Agência FAPESP – Researchers from Campinas are working to develop a viral vector capable of modifying the functions of certain defense cells, thereby stimulating the immune system to more effectively fight cancer.

The research is being carried out with FAPESP support at the Viral Vector Laboratory (LVV), which is housed at the Brazilian Biosciences National Laboratory of the Brazilian Center for Research in Energy and Materials (LNBio/CNPEM).

Preliminary data were presented by LVV coordinator Marcio Chaim Bajgelman at the 29th Annual Meeting of the Brazilian Federation of Experimental Biology Societies (FESBE) held in August in Caxambu (MG).

“In addition to the genetic mutation that triggers cancer, there is a series of other events in the body that occur in parallel, and these events can promote or inhibit the proliferation of cancer cells. One of these events is the patient’s own immune response, which we are attempting to modulate,” Bajgelman told Agência FAPESP.

According to the researcher, data from the scientific literature indicate that carriers of cancer often present elevated levels of regulatory T (Treg) cells, a type of lymphocyte that functions by inhibiting the proliferation of other lymphocytes that could attack the tumor cells.

In a physiological situation, Treg cells have an important mission to balance the immune system so that the body’s tissues are not attacked unnecessarily. However, in people with cancer, Bajgelman explained, these cells could actually help protect the tumor.

“Tumor cells produce substances that attract all types of T cells. When Treg cells migrate to the site of the tumor, they interact with so-called CD4 effector T cells and disarm them. If they are able to inhibit Treg cell activation, or even convert them to CD4 effector T cells, we could enable anti-tumor immunity,” Bajgelman said.

The biggest challenge for this proposed therapy, according to Bajgelman, is being able to distinguish between a Treg and a CD4 effector T cell. The two types of lymphocytes are morphologically similar and even have the same marker on the surface of the cell membrane: the CD25 receptor.

“There are Treg cell inhibition strategies that use antibodies against the CD25 receptor. However, this approach inhibits Treg cells as well as CD4 effector T cells. In this case, only the T CD8 lymphocytes remain active because they also have antitumoral activity. The scientific literature includes controversial results regarding the effectiveness of this type of therapy. We are trying to inhibit the Treg cells in a more selective manner,” Bajgelman explained.

The solution the researchers found was to select FOXP3 [forkhead box protein p3] as the target of the viral vector. FOXP3 is a transcription factor found in the nucleus of the Treg cells and is responsible for the immunosuppressive phenotype.

The idea is to insert into a modified virus an interfering RNA that is capable of preventing FOXP3 expression in Treg cells, thus nullifying their immunosuppressive activity. To do this, the researchers selected a lentiviral vector that is derived from human HIV but does not have the power to replicate or cause disease.

“Into this vector, we inserted an interfering RNA capable of binding exclusively to messenger RNA encoding FOXP3. To further increase the specificity of the therapy, we also inserted a promoter that will ensure that this interfering RNA targets only the cells that express FOXP3. Finally, we altered the capsid [outside part] of the virus so that it only targets CD4 T lymphocytes,” he explained.

Initial testing

In initial experiments conducted in vitro using cell cultures isolated from mouse spleens, the interfering RNA designed by the LVV team was able to inhibit FOXP3 expression by nearly 80%.

Functional tests were then run to determine whether the FOXP3 inhibition caused by the viral vector could in fact help the proliferation of CD4 effector T cells.

“We marked the CD4 effector T cells with a fluorescent dye and induced proliferation. As they divided, the intensity of the dye diminished,” the researcher explained.

When Treg cells were present on the culture plate, the proliferation rate of the CD4 effector T cells diminished by nearly 20%. When the viral vector was used, however, the scientists noted that proliferation rates returned to the levels observed in the control group.

The scientists from the LVV now plan to improve the interfering RNA and the promoter that will be inserted into the viral vector to make the action even more specific. When this step has been completed, initial testing on animals can begin.

“The current therapeutic arsenal for oncologists has few options for treating metastasis. If the results of our research are positive, we will have a new tool that could be used alone or with other treatments. It could help to reduce chemotherapy doses,” explained Bajgelman.