A lasting impression: CMV exposure imprints donor T cell phenotypes

From the Hill Lab, Clinical Research Division

Allogeneic hematopoietic stem cell transplantation (SCT) is a procedure in which the bone marrow from a healthy donor is used to reconstitute the hematopoietic system (including the cells of the immune system) of a patient whose blood cells have become compromised by cancer, cancer therapies, or other blood disorders. Unfortunately, these treatments are associated with risks of immune-related complications, including graft versus host disease, immune dysfunction, and opportunistic infections. Cytomegalovirus (CMV) infection is a significant risk for SCT recipients. An extremely common virus, CMV infects more than half of the population at some point in their lives, but rarely causes symptoms outside of immunocompromised individuals. After infection, the virus becomes latent within the host, and can be reactivated in the context of immune deficiency. Thus, SCT patients are commonly administered antiviral agents to prevent and treat patient- or donor-derived CMV reactivation.

It has been found that CMV-naïve, or seronegative, SCT recipients are at higher risk for complications after receiving bone marrow from CMV-experienced, or seropositive, donors compared to other recipient/donor combinations. Interestingly, increased levels of risk in this setting persist independently of viral reactivation, leading researchers to hypothesize that CMV exposure may compromise long-lasting immunity to other pathogens that impair long-term SCT outcomes. Members of the Hill Laboratory in the Fred Hutch Clinical Research Division, led by research associate Dr. Albert Yeh, in collaboration with fellow Cancer Consortium members Dr. Evan Newell (Vaccine and Infectious Disease and Public Health Sciences Divisions, Fred Hutch) and Dr. Stephanie Lee (Clinical Research Division, Fred Hutch), analyzed the relationship between recipient/donor serostatus and immune composition. Their work, recently published in Blood, reveals that an unusual cytolytic memory CD4+ T cell subset is stably enriched in seropositive transplants, and is associated with decreased T cell receptor repertoire diversity and reductions in antigen presenting cells.

Linking a new CMV-specific cytolytic memory CD4+ T cell subset to impaired immunity and survival in stem cell transplant recipients.
Linking a new CMV-specific cytolytic memory CD4+ T cell subset to impaired immunity and survival in stem cell transplant recipients. Image provided by Dr. Yeh.

T cells represent a major class of adaptive immune cells important for shaping and mediating antiviral immune responses. T cells detect the presence of viruses through their T cell receptors (TCRs), highly diverse surface receptors that are the products of genes that undergo randomized DNA rearrangements during T cell development. Each newly developed T cell carries a unique TCR capable of recognizing a specific molecular antigen, and, upon its activation and subsequent division, its progeny will inherit the same TCR sequence, forming a T cell ‘clone’. The clonal diversity and content of the T cell pool dictates the capacity of the adaptive immune system to recognize and respond to a wide variety of foreign invaders, and Fred Hutch researchers have identified TCR diversity as a measure for immune recovery following SCT.

In order to determine the relationship between SCT donor CMV seropositivity and immune composition, the Hill Lab analyzed blood samples collected from SCT patients of different recipient/donor serostatus combinations approximately one year after transplant, using a panel of T cell markers. Interestingly, a single T cell subset, characterized by high expression of CD4 and CD57 and low CD27 expression, was present in different proportions in blood samples derived from CMV seropositive vs. seronegative transplants. These cells were significantly enriched in seropositive transplants, even in the absence of CMV reactivation, and persisted for years after transplantation irrespective of whether patients received additional immunosuppressive therapy. CD57+/CD27- cells made up a larger fraction of the T cell compartment in seropositive transplants compared to healthy seropositive individuals, suggesting an expansion of this subset in the context of SCT. This subset exhibited an unusual phenotype of activated Th1 effector memory CD4+ T cells that also expressed high levels of cytolytic molecules normally associated with CD8+ T cells, suggesting that they may participate in responses against CMV. Indeed, CD57+/CD27- CD4+ T cells were preferentially reactive to CMV-derived antigens compared to the remainder of the T cell compartment.

Next, the group examined indicators of systemic immune modulation to identify possible mechanistic explanations for poor transplant outcomes from CMV seropositive donor grafts. Reasoning that expansion of these CMV-reactive T cells during virus exposure might result in constriction of the immune repertoire, the researchers performed TCR sequencing on patient-derived blood samples. In support of this hypothesis, they observed a significant inverse relationship between the prevalence of CD57+/CD27- CD4+ T cells and TCR diversity. Finally, the group demonstrated that this T cell subset was associated with reductions in MHC class II+ antigen presenting cell (APC) subsets, an important class of immune cells that initiate T cell responses to pathogens. Notably, the monocytic subset of APCs most affected are a target for CMV infection that act as a trojan horse to disseminate virus throughout the body.

This study identified correlates between SCT donor CMV serostatus, altered T cell phenotypes and diversity, and impairment in antigen presenting cell function. “While efforts at preventing CMV reactivation and disease have improved over the past decades, our work provides insight into the long-term immunologic impact that CMV exposure has on allogeneic transplants and how it can have a detrimental effect even in the absence of viral reactivation,” said Dr. Yeh. “This provides a stronger rationale for using CMV negative stem cell donors for CMV negative recipients.”  Moving forward, the group plans to understand the role of these CD4+ T cells in controlling the dissemination of CMV by patrolling monocytes and resulting impairments in antigen presentation that likely compromise immunity to other pathogens. “One direction we are interested in taking this work is to further elucidate the function of these CD57+ cytolytic CD4+ T cells,” explained Dr. Yeh. “Do these cells provide a necessary or important function in CMV immunity, or are they a byproduct of our immune system's response to latent CMV infection? We are keen on demonstrating the functional ability of these CD57+ CD4+ T-cells in modulating MHC class II presenting cells and proving, for example, that they are directly cytotoxic.” This work offers important insights into potential mechanisms underlying poor patient outcomes, and possible points of therapeutic intervention following stem cell transplantation.

This work was funded by the National Institutes of Health and the National Cancer Institute.

UW/Fred Hutch Cancer Consortium members Evan Newell, Stephanie Lee, and Geoffrey Hill contributed to this work.

Yeh AC, Varelias A, Reddy A, Barone SM, Olver S, Chilson K, Onstad L, Ensbey KS, Henden AS, Samson L, Jaeger CA, Bi T, Dahlman K, Kim TK, Zhang P, Degli-Esposti MA, Newell EW, Jagasia M, Irish JM, Lee SJ, Hill GR. CMV exposure drives long-term CD57+ CD4 memory T cell inflation following allogeneic stem cell transplant. Blood. 2021 Jun 3:blood.2020009492. doi: 10.1182/blood.2020009492. Epub ahead of print. PMID: 34115118.