Short answer: CMV (Cytomegalovirus – Wikipedia) infects most human beings at some point in their life. Human anti-CMV T cell responses are thus a convenient in vitro antigen-specific tool to assess checkpoint inhibitor mAb (monoclonal antibody) functional capability, specifically, whether they function as they should and thus by extension, could reverse human cancer antigen-specific T cell inhibition.

Slightly longer answer

Brief Explanation For Rationale Of Checkpoint Inhibitors

Activated T cells express the PD-1 molecule on their surface. T cell PD-1 binding to PD-L1 expressed by other cells sends an inhibitory signal into the T cell. Such T cells stop proliferating and secreting cytokines, meaning PD-1-PD-L1 binding essentially inhibits T cell effector function.

Many cancer cells highly express PD-L1. Checkpoint inhibitor mAbs that selectively target PD-1 (Nivolumab – Wikipedia and Pembrolizumab – Wikipedia) and PD-L1 (Atezolizumab – Wikipedia) molecules prevent PD-1-PD-L1 interaction. This is akin to releasing the brakes off of previously activated and presently inhibited T cells.

Checkpoint inhibitors used in Cancer immunotherapy – Wikipedia imply cancer antigen-specific T cells exist but are inhibited by molecules such as PD-1 and CTLA-4 and that blocking them releases the full tumor-killing potential of such T cells. Checkpoint inhibitors unblock other T cells as well, meaning autoimmunity and other tissue pathologies are also possible.

CMV Recall Assay To Assess Functional Capability Of Checkpoint Inhibitor mAbs

A checkpoint inhibitor mAb such as anti-PD-1 Nivolumab can bind PD-1 expressed by any T cell. How to assess if such binding did indeed lift the brakes off of that T cell as it should? Knowing a T cell’s antigenic specificity makes it a piece of cake to assess changes in its responsiveness. For example, in basic immunology, often experiments are done with genetically engineered TCR – Wikipedia transgenic mice having T cells with a single known, i.e., monoclonal, antigenic specificity. The antigenic peptide specificity of all the T cells in such a mouse is the same. Just culture such T cells with APCs (Antigen-presenting cell – Wikipedia) pulsed with the antigenic peptide they’re specific for in the presence or absence of a checkpoint inhibitor mAb.

Obviously, this is far from feasible for assessing human T cell responses. What type of antigen could then be used to assess functional alterations in human T cell responsiveness? This is especially an issue when using checkpoint inhibitor mAbs in cancer immunotherapy, where cancer-specific T cell antigens often remain unknown. How to confirm functional capability of checkpoint inhibitor mAbs, confirm that a new batch can lift the brakes off of previously activated but currently inhibited T cells as it should? That’s where in vitro assays such as the non-antigen-specific MLR (Mixed lymphocyte reaction – Wikipedia), Superantigen – Wikipedia SEB (Enterotoxin type B – Wikipedia) stimulation and the antigen-specific CMV recall assay enter the picture.


  • Chronically infects most human beings at some point in their life (1).
  • Estimated ~60% and ~90% US prevalence in those >/=6 and >/=80 years of age, respectively, from 1988 to 1994 (2).
  • Elicits strong T cell immune responses and plenty is known about the features of effective human anti-CMV immunity (3).
  • For example, CMV’s pp65 protein is a major target of human CD4 and CD8 T cell responses (4, 5, 6, 7, 8).

Such features make assessing changes in human anti-CMV T cell responses a handy tool to monitor functional capability of checkpoint inhibitor mAbs. See an example of such assessment below from 9.


1. Tirumalai Kamala’s answer to What do we know about the function of viruses in the microbiome?

2. Staras, Stephanie AS, et al. “Seroprevalence of cytomegalovirus infection in the United States, 1988–1994.” Clinical Infectious Diseases 43.9 (2006): 1143-1151. https://www.researchgate.net/pro…

3. Gamadia, Laila E., et al. “Primary immune responses to human CMV: a critical role for IFN-γ–producing CD4+ T cells in protection against CMV disease.” Blood 101.7 (2003): 2686-2692. https://www.researchgate.net/pro…

4. Grefte, J. M. M., et al. “The lower matrix protein pp65 is the principal viral antigen present in peripheral blood leukocytes during an active cytomegalovirus infection.” Journal of general virology 73.11 (1992): 2923-2932.

5. Khattab, Barbara Anna‐Maria, et al. “Three T‐cell epitopes within the C‐terminal 265 amino acids of the matrix protein pp65 of human cytomegalovirus recognized by human lymphocytes.” Journal of medical virology 52.1 (1997): 68-76.

6. Gyulai, Zsofia, et al. “Cytotoxic T lymphocyte (CTL) responses to human cytomegalovirus pp65, IE1-Exon4, gB, pp150, and pp28 in healthy individuals: reevaluation of prevalence of IE1-specific CTLs.” Journal of Infectious Diseases 181.5 (2000): 1537-1546. https://www.researchgate.net/pro…

7. Gibson, Laura, et al. “Human cytomegalovirus proteins pp65 and immediate early protein 1 are common targets for CD8+ T cell responses in children with congenital or postnatal human cytomegalovirus infection.” The Journal of Immunology 172.4 (2004): 2256-2264. https://www.researchgate.net/pro…

8. Sylwester, Andrew W., et al. “Broadly targeted human cytomegalovirus-specific CD4+ and CD8+ T cells dominate the memory compartments of exposed subjects.” Journal of Experimental Medicine 202.5 (2005): 673-685. https://pdfs.semanticscholar.org…

9. Wang, Changyu, et al. “In vitro characterization of the anti-PD-1 antibody nivolumab, BMS-936558, and in vivo toxicology in non-human primates.” Cancer immunology research 2.9 (2014): 846-856. http://cancerimmunolres.aacrjour…