Cancers can harvest energy from acetate

Acetyl-CoA plays a critical role in cell metabolism. In normoxic condition, acetyl-CoA is derived from pyruvate. However, in hypoxic condition as in many cancer tissue experience, pyruvate is preferentially converted into lactate. So, how cancer cells generate sufficient level of acetyl-CoA to fuel the growth?

Study in journal Cell suggests that the source of acetyl-CoA in cancer cells is a little known metabolite acetate. 

Initially, using hepatocyte cell line, HepG2, and siRNA approach, the authors, led by Drs. Steven McKnight and Benjamin Tu at UT Southwestern Medical Center, Dallas, found that out 3 iso-enzymes capable of acetate conversion into acetyl-CoA and its incorporation into lipid or protein synthesis, iso-form 2 (acetyl-CoA synthetase enzyme 2) was mostly responsible for this action in an vitro assay. 

Similar results were obtained when cells harvested from mice deficient in both or one allele of iso-form 2 were tested.

Interestingly, examination of several tumor cell lines revealed that inhibition of iso-form 2 had a major impact on acetate incorporation into lipids or proteins.

PET image analysis confirmed that tumors could efficiently incorporate acetate.

More importantly, using inducible or spontaneous liver tumor models, the authors observed that iso-form 2-deficient mice showed diminished tumor burden (both in size and quantity).

Finally, analysis of human tissue samples showed high level of expression of iso-form 2 in cancer tissue compared to normal cells.

In summary, the results of this study, and another study on human glioblastoma, led by Elizabeth Maher and Robert Bachoo (from the same research center), suggest new treatment path for cancer therapy.

David Usharauli

10 in a Million: making tumors visible to immune system

Another interesting study about tumor immunology from Nature Medicine. 

This time, the authors, led by Ton Schumacher at the Netherlands Cancer Institute, have examined cancer proteome (mutanome) from 5 melanoma patients using whole exome and RNA sequencing coupled with in vitro T cell assay to identify (1) an entire set of non-synonymous mutations within expressed genes and (2) immunogenic neo-antigens.

On average the authors found that melanoma patients expressed ~ 150 mutations per tumor. To identify immunogenic epitopes, 31-mer peptides corresponding to mutated portion of the proteins were synthesized, loaded into autologous, immortalized (via BCL6/BCL-XL transduction) B cells, serving as APCs, and cultured them with in vitro expanded tumor-derived CD4 T cells. With these methods, the authors were able to identify several tumor-specific, neo-antigens.

Importantly, sensitivity of tumor-associated CD4 T cells to mutant neo-antigens (open symbols) were on average 100-fold higher as compared to their wild-type variants (closed symbols), implying that mutant epitopes were quite immunogenic.

Interestingly, based on analyses of cancer mutation burden and clinical immune response, the authors speculated that mutation rates of at least 10 per megabase of coding genome provides sufficient load for formation of immunogenic neo-epitopes detected by immune system (for example, one patient, NKIRTIL045 had less than 10 mutations per megabase and did not show CD4 T cell response to mutated epitopes; see Fig. 1).  

In summary, all these recent studies and successes in cancer immunology clearly indicate that President Nixon's declaration of war on cancer has finally started to pay off.

David Usharauli

Enigma of the peripheral tolerance: CTLA4+ anergic CD8 T cells

When Sir Burnet first proposed clonal-selection theory of everything to explain how immune system could detect non-self antigens while at the same time avoiding autoimmunity, he probably had no idea that his theory would require major and multiple modifications to accommodate new data.

This new study from journal Science is another piece in the puzzle. This work, led by Shimon Sakaguchi at Osaka University, studied self-reactive CD8 T cells from the healthy humans or from vitiligo patients's blood.

The authors studied self-antigen, melanin-specific CD8 T cells. In T cell proliferation assay, Melan-A specific CD8 T cells, almost undetectable before stimulation, undergo robust, multiple-round proliferation and expand to sizable population. However, in presence of regulatory T cells, Melan-A specific CD8 T cells undergo single-round, abortive proliferation.

Tetramer staining indicated that in presence of regulatory T cells, Melan-A specific CD8 T cells mostly consisted of low-affinity clones and displayed anergic phenotype with low cytokine expression (however, please note that small population of CD8 T cells displayed high affinity binding to tetramers even in presence of regulatory T cells. This suggest to me that regulatory T cells suppress proliferation of these high-affinity Melan-A specific CD8 T cells).

Surface staining showed that anergic CD8 T cells generated in presence of T regs express high levels of CTLA-4 and CCR7.

Direct ex vivo analysis of healthy human CD8 T cells showed that naive CD8 T cells consisted of CTLA4+ and CTLA4- population. Antigen-specific or non-specific stimulation of these two population showed that CTLA4+ naive CD8 T cells were (1) enriched in self-specific CD8 T cells, (2) displayed anergic phenotype (abortive proliferation) and (3) preferentially undergo apoptosis in mixed culture.

In summary, the authors showed that healthy human blood contains two types of "naive" CD8 T cell populations: naive, CTLA4neg functionally-competent CD8 T cells and anergic, CTLA4pos functionally-impaired CD8 T cell population.

It appears, though there is no direct evidence, that T regs control self-antigen specific CTLA4+ anergic CD8 T cell development from high-affinity clones. If the model is accurate, then it is not clear why healthy human blood still contains high affinity, self-antigen specific CTLA4- CD8 T cell population (see Fig. 1b tetramer staining. I wonder whether small population of high-affinity CD8 T cells in Treg culture express CTLA4). 

David Usharauli