• The nicotinamide adenine dinucleotide (NAD+) precursor NMN improves hepatitis B virus-specific immune cell (CD8 T cell) antiviral activation.
  • Exhausted hepatitis B-specific CD8 T cells with impaired function exhibit high levels of DNA damage and defective DNA repair mechanisms.
  • In their exhausted state, CD8 T immune cells display higher NAD+-consuming CD38 enzyme levels and lower antiviral protein production.

Hepatitis B typically resolves within six months for most people infected, but chronic hepatitis B that doesn’t go away can occur due to CD8 T cell exhaustion. CD8 T cell exhaustion results from high levels of exposure to a pathogen like the hepatitis B virus, triggering T cell DNA damage and a dysfunctional antiviral response. Identifying the cellular processes behind CD8 T cell exhaustion can help with developing therapies for chronic hepatitis B. Some researchers have recently proposed that NAD+ depletion underlies T cell exhaustion.

Published in the Journal of Hepatology, Fisicaro and colleagues from the University of Parma in Italy demonstrate that treating CD8 T cells specific for the hepatitis B virus with NMN improves their production of antiviral proteins – cytokines. Exhausted hepatitis B-specific CD8 T cells were found to have high levels of DNA damage along with dysfunctional DNA repair mechanisms. Findings from the study suggest that NMN and/or CD38 inhibitors can be used to revamp exhausted T cells to ward off chronic hepatitis B, along with potentially other viral infections.

NMN Restores Immune Cell Antiviral Activation

Because Fisicaro and colleagues hypothesized that restoring NAD+ levels with NMN could revamp exhausted CD8 T cells, they isolated these immune cells from chronic hepatitis B patients for testing. Upon treatment with NMN, the cells expressed more antiviral cytokines, specifically a 2.7-fold increase for cytokine interferon gamma (IFN-𝛾), suggesting that NMN restores the CD8 T cells’ antiviral properties.

(Montali et al., 2023 | Journal of Hepatology) NMN restores immune cell antiviral cytokine production. Cellular cytokines listed on the X-axis significantly increased following immune cell treatment with NMN and hepatitis B viral proteins (black bars) compared to those treated with hepatitis B viral proteins alone (gray bars). Med + Pep= Cell culture medium with viral peptide stimulation; Med + Pep + NMN= Cell culture medium with viral peptide stimulation and NMN

To examine the cellular damage that goes along with T cell exhaustion in chronic hepatitis B infection, Fisicaro and colleagues compared DNA damage in hepatitis B-specific T cells with influenza (FLU)-specific T cells. The researchers found significantly more DNA damage in the hepatitis B-specific T cells. They also found that treating the cells with a DNA damage-inducing molecule (etoposide) elicited a trend toward a lower DNA damage response compared to FLU-specific T cells. Since DNA damage responses like the one mediated by PARPs require NAD+, this data suggests that NAD+ deficiency is tied to higher levels of DNA damage in exhausted hepatitis B-specific CD8 T cells.

(Montali et al., 2023 | Journal of Hepatology) Hepatitis B-specific CD8 T cells have high levels of DNA damage and a weak DNA damage response to a DNA damaging molecule (etoposide). Left) Compared to FLU-specific CD8 T cells (FLU HEALTHY), those specific for hepatitis B (HBV CHRONIC) exhibit higher levels of markers for DNA damage (phosphoH2AX). Right) A molecule used to induce DNA damage (etoposide) elicits a trend toward lower DNA damage response in hepatitis B-specific immune cells (HBV CHRONIC) compared to FLU-specific immune cells (FLU HEALTHY), suggesting lower NAD+ required for DNA repair.

To explore further how NAD+ repletion with NMN improves exhausted CD8 T cell antiviral cytokine production, Fisicaro and colleagues measured how high levels of the NAD+-consuming CD38 enzyme relate to cytokine levels. They measured one of the primary cytokines – IFN-𝛾 – and found that high CD38 enzyme levels are associated with low IFN-𝛾 levels. These data provide evidence that high CD38 enzyme levels can lead to lower antiviral cytokine production. This also lends credulity to the assertion that low NAD+ levels play an important role in chronic hepatitis B pathology since high CD38 levels would theoretically lower cellular NAD+.

(Montali et al., 2023 | Journal of Hepatology) Higher immune cell CD38 enzyme levels are associated with lower antiviral cytokine levels. Higher percentages of immune cells with elevated CD38 (%CD38hi) are associated with a lower probability of expressing the antiviral cytokine IFN-𝛾 (%IFN-𝛾+).

“Our data show increased DNA damage with limited activation of the DNA repair machinery in [hepatitis B virus]-specific CD8 T cells from [chronic hepatitis B] patients,” say Fisicaro and colleagues. “This strongly suggests that NAD-consuming enzymes, particularly overexpressed CD38, may play a pivotal role in NAD depletion. Reconstitution of many interconnected intracellular functions by NMN supplementation indicates that NAD depletion likely represents an important determinant of T cell exhaustion.”

NMN May Revamp Immune Cells in Multiple Types of Infections

The study provides evidence that the hepatitis B virus triggers higher CD38 enzyme activation, leading to lower NAD+ levels. Since DNA repair mechanisms like PARPs require NAD+ for their function, this could help explain why exhausted hepatitis B virus-specific T cells have more DNA damage. This could also help explain why DNA repair mechanisms don’t work as well in exhausted hepatitis B-specific CD8 T cells.

Treating CD8 T immune cells with the NAD+ precursor NMN restores antiviral cytokine production, suggesting that NAD+ repletion reconstitutes their function. NMN’s capability to revamp immune cell function provides support that NAD+ depletion contributes to CD8 T cell exhaustion and dysfunction. Whether NMN provides beneficial effects in exhausted CD8 T cells in other types of infections should be explored. If NMN can restore exhausted T cells in other infections, it may be the case that NAD+ makes it easier for immune cells to fight other types of infection, also.