Progress and Challenges Toward Generation and Maintenance of Long-Lived Memory T Lymphocyte Responses During COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causing the coronavirus disease 2019 (COVID-19) pandemic is a serious global threat until we identify the effective preventive and therapeutic strategies. SARS-CoV-2 infection is characterized by various immunopathological consequences including lymphocyte activation and dysfunction, lymphopenia, cytokine storm, increased level of neutrophils, and depletion and exhaustion of lymphocytes. Considering the low level of antibody-mediated protection during coronavirus infection, understanding the role of T cell for long-term protection is decisive. Both CD4⁺ and CD8⁺ T cell response is imperative for cell-mediated immune response during COVID-19. However, the level of CD8⁺ T cell response reduced to almost half as compared to CD4⁺ after 6 months of infection. The long-term protection is mediated via generation of immunological memory response during COVID-19. The presence of memory CD4⁺ T cells in all the severely infected and recovered individuals shows that the memory response is predominated by CD4⁺ T cells. Prominently, the antigen-specific CD4⁺ and CD8⁺ T cells are specifically observed during day 0 to day 28 in COVID-19-vaccinated individuals. However, level of antigen-specific T memory cells in COVID-19-vaccinated individuals defines the long-term protection against forthcoming outbreaks of SARS-CoV-2.

T follicular helper and memory cell responses and the mTOR pathway in murine heart transplantation

BACKGROUND

The mammalian target of rapamycin (mTOR) inhibitors are valuable immunosuppressants in clinical transplantation; however, the mTOR regulation of allogeneic T-cell responses is not fully understood yet. Therefore, the objective of this study is to investigate the effects of T-cell-specific mTOR deletion on the allogeneic T-cell responses and heart transplant survival.

METHODS

BALB/c heart allografts, with or without BALB/c skin sensitization, were transplanted in the wild-type C57BL/6, Mtor^fl/flCd4-Cre, Stat3^fl/flCd4-Cre, and Mtor^fl/flStat3^fl/flCd4-Cre mice. Graft survival and histology, as well as T-cell frequencies and phenotypes, were evaluated after transplantation.

RESULTS

In the absence of donor skin sensitization, long-term heart allograft survival was achieved in the Mtor^fl/flCd4-Cre recipients, which was associated with significantly decreased frequencies of CD62L^-CD44⁺ effector T cells and BCL-6⁺CXCR5⁺ T follicular helper (Tfh) cells in the periphery. Long-term heart allograft survival was also achieved in the donor skin-sensitized Mtor^fl/flStat3^fl/flCd4-Cre mice, whereas the heart allograft survival was prolonged in the donor skin-sensitized Mtor^fl/flCd4-Cre and Stat3^fl/flCd4-Cre mice.

CONCLUSIONS

mTOR is required for Tfh cell response in murine heart transplantation. T-cell-specific deletion of both mTOR and Stat3 abrogates the memory response to heart transplants. These findings help us to better understand the molecular mechanisms underlying the T cell immunity to transplanted organs.

The Effect of CpG Sequences on Capsid-Specific CD8+ T Cell Responses to AAV Vector Gene Transfer

Transfer of genes by adeno-associated virus (AAV) vectors is benefiting patients with particular genetic defects. Challenges remain by rejection of AAV-transduced cells, which may be caused by CD8⁺ T lymphocytes directed to AAV capsid antigens. Reducing the number of CpG motifs from the genome of AAV vectors reduces expansion of naive T cells directed against an epitope within the capsid. In contrast, AAV capsid-specific memory CD8⁺ T cells respond more vigorously to AAV vectors lacking CpG motifs than to those with CpG motifs presumably reflecting dampening of T cell expansion by cytokines from the innate immune system. Depending on the purification method, AAV vector preparations can contain substantial amounts of empty AAV particles that failed to package the genome. Others have used empty particles as decoys to AAV-neutralizing antibodies. We tested if empty AAV vectors given alone or mixed with genome-containing AAV vectors induce proliferation of naive or memory CD8⁺ T cells directed to an antigen within an AAV capsid. Naive CD8⁺ T cells failed to respond to empty AAV vectors, which in contrast induced expansion of AAV-specific memory CD8⁺ T cells.