Adoptive CD8 T cell control of pathogens cannot be improved by combining protective epitope specificities

Therapeutic Vaccination of Hematopoietic Cell Transplantation Recipients Improves Protective CD8 T-Cell Immunotherapy of Cytomegalovirus Infection

Reactivation of latent cytomegalovirus (CMV) endangers the therapeutic success of hematopoietic cell transplantation (HCT) in tumor patients due to cytopathogenic virus spread that leads to organ manifestations of CMV disease, to interstitial pneumonia in particular. In cases of virus variants that are refractory to standard antiviral pharmacotherapy, immunotherapy by adoptive cell transfer (ACT) of virus-specific CD8+ T cells is the last resort to bridge the "protection gap" between hematoablative conditioning for HCT and endogenous reconstitution of antiviral immunity. We have used the well-established mouse model of CD8+ T-cell immunotherapy by ACT in a setting of experimental HCT and murine CMV (mCMV) infection to pursue the concept of improving the efficacy of ACT by therapeutic vaccination (TherVac) post-HCT. TherVac aims at restimulation and expansion of limited numbers of transferred antiviral CD8+ T cells within the recipient. Syngeneic HCT was performed with C57BL/6 mice as donors and recipients. Recipients were infected with recombinant mCMV (mCMV-SIINFEKL) that expresses antigenic peptide SIINFEKL presented to CD8+ T cells by the MHC class-I molecule Kb. ACT was performed with transgenic OT-I CD8+ T cells expressing a T-cell receptor specific for SIINFEKL-Kb. Recombinant human CMV dense bodies (DB-SIINFEKL), engineered to contain SIINFEKL within tegument protein pUL83/pp65, served for vaccination. DBs were chosen as they represent non-infectious, enveloped, and thus fusion-competent subviral particles capable of activating dendritic cells and delivering antigens directly into the cytosol for processing and presentation in the MHC class-I pathway. One set of our experiments documents the power of vaccination with DBs in protecting the immunocompetent host against a challenge infection. A further set of experiments revealed a significant improvement of antiviral control in HCT recipients by combining ACT with TherVac. In both settings, the benefit from vaccination with DBs proved to be strictly epitope-specific. The capacity to protect was lost when DBs included the peptide sequence SIINFEKA lacking immunogenicity and antigenicity due to C-terminal residue point mutation L8A, which prevents efficient proteasomal peptide processing and binding to Kb. Our preclinical research data thus provide an argument for using pre-emptive TherVac to enhance antiviral protection by ACT in HCT recipients with diagnosed CMV reactivation.

Refining human T-cell immunotherapy of cytomegalovirus disease: a mouse model with 'humanized' antigen presentation as a new preclinical study tool

With the cover headline 'T cells on the attack,' the journal Science celebrated individualized cancer immunotherapy by adoptive transfer of T cells as the 'Breakthrough of the Year' 2013 (J. Couzin-Frankel in Science 342:1432-1433, 2013). It is less well recognized and appreciated that individualized T cell immunotherapy of cytomegalovirus (CMV) infection is approaching clinical application for preventing CMV organ manifestations, interstitial CMV pneumonia in particular. This coincident medical development is particularly interesting as reactivated CMV infection is a major viral complication in the state of transient immunodeficiency after the therapy of hematopoietic malignancies by hematopoietic cell transplantation (HCT). It may thus be attractive to combine T cell immunotherapy of 'minimal residual disease/leukemia (MRD)' and CMV-specific T cell immunotherapy to combat both risks in HCT recipients simultaneously, and ideally with T cells derived from the respective HLA-matched HCT donor. Although clinical trials of human CMV-specific T cell immunotherapy were promising in that the incidence of virus reactivation and disease was found to be reduced with statistical significance, animal models are still instrumental for providing 'proof of concept' by directly documenting the prevention of viral multiple-organ histopathology and organ failure under controlled conditions of the absence versus presence of the therapy, which obviously is not feasible in an individual human patient. Further, animal models can make predictions regarding parameters that determine the efficacy of T cell immunotherapy for improved study design in clinical investigations, and they allow for manipulating host and virus genetics. The latter is of particular value as it opens the possibility for epitope specificity controls that are inherently missing in clinical trials. Here, we review a recently developed new mouse model that is more approximated to human CMV-specific T cell immunotherapy by 'humanizing' antigen presentation using antigenically chimeric CMV and HLA-transgenic mice to allow for an in vivo testing of the antiviral function of human CMV-specific T cells. As an important new message, this model predicts that T cell immunotherapy is most efficient if CD4 T cells are equipped with a transduced TCR directed against an epitope presented by MHC/HLA class-I for local delivery of 'cognate' help to CD8 effector T cells at infected MHC/HLA class-II-negative host tissue cells.

Spatial distribution and structural arrangement of a murine cytomegalovirus glycoprotein detected by SPDM localization microscopy

Novel approaches of localization microscopy have opened new insights into the molecular nano-cosmos of cells. We applied a special embodiment called spectral position determination microscopy (SPDM) that has the advantage to run with standard fluorescent dyes or proteins under standard preparation conditions. Pointillist images with a resolution in the order of 10 nm can be obtained by SPDM. Therefore, vector pEYFP-m164, encoding the murine cytomegalovirus glycoprotein gp36.5/m164 fused to enhanced yellow fluorescent protein, was transiently transfected into COS-7 cells. This protein shows exceptional intracellular trafficking dynamics, moving within the endoplasmic reticulum (ER) and outer nuclear membrane. The molecular positions of gp36.5/m164 were visualized and determined by SPDM imaging. From the position point patterns of the protein molecules, their arrangements were quantified by next neighbour distance analyses. Three different structural arrangements were discriminated: (a) a linear distribution along the membrane, (b) a highly structured distribution in the ER, and (c) a homogenous distribution in the cellular cytoplasm. The results indicate that the analysis of next neighbour distances on the nano-scale allows the identification and discrimination of different structural arrangements of molecules within their natural cellular environment.

Acute neonatal infections 'lock-in' a suboptimal CD8+ T cell repertoire with impaired recall responses

Microbial infection during various stages of human development produces widely different clinical outcomes, yet the links between age-related changes in the immune compartment and functional immunity remain unclear. The ability of the immune system to respond to specific antigens and mediate protection in early life is closely correlated with the level of diversification of lymphocyte antigen receptors. We have previously shown that the neonatal primary CD8+ T cell response to replication competent virus is significantly constricted compared to the adult response. In the present study, we have analyzed the subsequent formation of neonatal memory CD8+ T cells and their response to secondary infectious challenge. In particular, we asked whether the less diverse CD8+ T cell clonotypes that are elicited by neonatal vaccination with replication competent virus are ‘locked-in’ to the adult memory T cell, and thus may compromise the strength of adult immunity. Here we report that neonatal memory CD8+ T cells mediate poor recall responses compared to adults and are comprised of a repertoire of lower avidity T cells. During a later infectious challenge the neonatal memory CD8+ T cells compete poorly with the fully diverse repertoire of naïve adult CD8+ T cells and are outgrown by the adult primary response. This has important implications for the timing of vaccination in early life.

Newborns typically have a heightened sensitivity to infectious diseases, the reasons for which are not yet well understood. One contributing factor is the limited diversity of lymphocyte receptors early in life to recognize antigen and control infection. We have previously shown that antigen-specific CD8+ T cell repertoires are significantly constricted in neonates compared with adults. In this study, we addressed the question of whether the developmental stage of the host at the time of vaccination influences the composition of the memory CD8⁺ T cell repertoire and its ability to mount a robust response to subsequent infections. We observed that the antigen-specific T cell repertoires elicited in the context of an acute neonatal infection, that are less diverse and comprised of lower-avidity T cells, are partially ‘locked-in’ to the adult memory T cell repertoire. However, in the face of a secondary infectious challenge, naïve adult T cells outcompete the lower avidity neonatal memory T cells and raise the diversity of the overall CD8⁺ T cell response. These results have potential implications for the design of vaccines to be administered in early life.

Parameters determining the efficacy of adoptive CD8 T-cell therapy of cytomegalovirus infection

Reactivation of latent cytomegalovirus (CMV) in the transient state of immunodeficiency after hematopoietic cell transplantation (HCT) is the most frequent and severe viral complication endangering leukemia therapy success. By infecting the bone marrow (BM) stroma of the transplantation recipient, CMV can directly interfere with BM repopulation by the transplanted donor-derived hematopoietic cells and thus delay immune reconstitution of the recipient. Cytopathogenic virus spread in tissues can result in CMV disease with multiple organ manifestations of which interstitial pneumonia is the most feared. There exists a 'window of risk' between hematoablative treatment and reconstitution of antiviral immunity after HCT, whereby timely reconstitution of antiviral CD8 T cells is a recognized positive prognostic parameter for the control of reactivated CMV infection and prevention of CMV disease. Supplementation of endogenous reconstitution by adoptive cell transfer of 'ready-to-go' effector and/or memory virus epitope-specific CD8 T cells is a therapeutic option to bridge the 'window of risk.' Preclinical research in murine models of CMV disease has been pivotal by providing 'proof of concept' for a benefit from CD8 T-cell therapy of HCT-associated CMV disease (reviewed in Holtappels et al. Med Microbiol Immunol 197:125-134, 2008). Here, we give an update of our previous review with focus on parameters that determine the efficacy of adoptive immunotherapy of CMV infection by antiviral CD8 T cells in the murine model.

Recombinant mouse cytomegalovirus expressing a ligand for the NKG2D receptor is attenuated and has improved vaccine properties

Human CMV (HCMV) is a major cause of morbidity and mortality in both congenitally infected and immunocompromised individuals. Development of an effective HCMV vaccine would help protect these vulnerable groups. NK group 2, member D (NKG2D) is a potent activating receptor expressed by cells of the innate and adaptive immune systems. Its importance in HCMV immune surveillance is indicated by the elaborative evasion mechanisms evolved by the virus to avoid NKG2D. In order to study this signaling pathway, we engineered a recombinant mouse CMV expressing the high-affinity NKG2D ligand RAE-1γ (RAE-1γMCMV). Expression of RAE-1γ by MCMV resulted in profound virus attenuation in vivo and lower latent viral DNA loads. RAE-1γMCMV infection was efficiently controlled by immunodeficient hosts, including mice lacking type I interferon receptors or immunosuppressed by sublethal γ-irradiation. Features of MCMV infection in neonates were also diminished. Despite tight innate immune control, RAE-1γMCMV infection elicited strong and long-lasting protective immunity. Maternal RAE-1γMCMV immunization protected neonatal mice from MCMV disease via placental transfer of antiviral Abs. Despite strong selective pressure, the RAE-1γ transgene did not exhibit sequence variation following infection. Together, our results indicate that use of a recombinant virus encoding the ligand for an activating NK cell receptor could be a powerful approach to developing a safe and immunogenic HCMV vaccine.

Immunotherapy of viral infections

Among the microorganisms that cause diseases of medical or veterinary importance, the only group that is entirely dependent on the host, and hence not easily amenable to therapy via pharmaceuticals, is the viruses. Since viruses are obligate intracellular pathogens, and therefore depend a great deal on cellular processes, direct therapy of viral infections is difficult. Thus, modifying or targeting nonspecific or specific immune responses is an important aspect of intervention of ongoing viral infections. However, as a result of the unavailability of effective vaccines and the extended duration of manifestation, chronic viral infections are the most suitable for immunotherapies. We present an overview of various immunological strategies that have been applied for treating viral infections after exposure to the infectious agent.

Lessons learned from natural infection: focusing on the design of protective T cell vaccines for HIV/AIDS

CD8(+) cytotoxic T lymphocyte (CTL) responses are crucial in establishing the control of persistent virus infections. Population studies of HIV-1-infected individuals suggest that CD8(+) CTL responses targeting epitopes that take the greatest toll on virus replication are instrumental in immune control. A major question for vaccine design is whether incorporating epitopes responsible for controlling a persistent virus will translate into protection from natural infection or serve solely as a fail-safe mechanism to prevent overt disease in infected individuals. Here, we discuss qualitative parameters of the CD8(+) CTL response and mechanisms operative in the control of persistent virus infections and suggest new strategies for design and delivery of HIV vaccines.

CD8 T-cell-based immunotherapy of cytomegalovirus infection: "proof of concept" provided by the murine model

Adoptive transfer of antiviral effector or memory CD8 T cells is a therapeutic option for preventing acute cytomegalovirus (CMV) disease after primary or recurrent infection in immunocompromised recipients of hematopoietic stem cell transplantation (HSCT) aimed at curing hematopoietic malignancies. Preclinical research in murine models has demonstrated the power of CD8 T-cell-based preemptive immunotherapy and has encouraged clinical trials that gave promising results. The clinical evidence, however, is based primarily on statistical analyses indicating a reduced incidence of CMV-associated complications. Here, we will briefly review the data obtained from the murine model showing that CD8 T cells derived from CMV-immune donors and administered either as peptide-selected cytolytic T lymphocyte lines or after ex vivo purification by T-cell-receptor-specific cell sorting can indeed prevent CMV-mediated histopathology and multiple organ failure.