Direct presentation is sufficient for an efficient anti-viral CD8+ T cell response

Induction and function of virus-specific CD4+ T cell responses

CD4+ T cells - often referred to as T-helper cells - play a central role in immune defense and pathogenesis. Virus infections and vaccines stimulate and expand populations of antigen-specific CD4+ T cells in mice and in man. These virus-specific CD4+ T cells are extremely important in antiviral protection: deficiencies in CD4+ T cells are associated with virus reactivation, generalized susceptibility to opportunistic infections, and poor vaccine efficacy. As described below, CD4+ T cells influence effector and memory CD8+ T cell responses, humoral immunity, and the antimicrobial activity of macrophages and are involved in recruiting cells to sites of infection. This review summarizes a few key points about the dynamics of the CD4+ T cell response to virus infection, the positive role of pro-inflammatory cytokines in the differentiation of virus-specific CD4+ T cells, and new areas of investigation to improve vaccines against virus infection.

Reverse vaccinology: developing vaccines in the era of genomics

The sequence of microbial genomes made all potential antigens of each pathogen available for vaccine development. This increased by orders of magnitude potential vaccine targets in bacteria, parasites, and large viruses and revealed virtually all their CD4(+) and CD8(+) T cell epitopes. The genomic information was first used for the development of a vaccine against serogroup B meningococcus, and it is now being used for several other bacterial vaccines. In this review, we will first summarize the impact that genome sequencing has had on vaccine development, and then we will analyze how the genomic information can help further our understanding of immunity to infection or vaccination and lead to the design of better vaccines by diving into the world of T cell immunity.

Analysis of A47, an immunoprevalent protein of vaccinia virus, leads to a reevaluation of the total antiviral CD8+ T cell response

Vaccinia virus (VACV) is the prototypic orthopoxvirus and was the live vaccine used to eradicate smallpox. In addition, VACV is a possible vector for recombinant vaccines. Despite these reasons for study, the roles of many VACV genes are unknown, and some fundamental aspects, such as the total size of immune responses, remain poorly characterized. VACV gene A47L is of interest because it is highly transcribed, has no sequence similarity to any nonpoxvirus gene, and contains a larger-than-expected number of CD8(+) T cell epitopes. Here it is shown that A47L is not required for growth in vitro and does not contribute to virulence in mice. However, we confirmed that this one protein primes CD8(+) T cells to three different epitopes in C57BL/6 mice. In the process, one of these epitopes was redefined and shown to be the most dominant in A47 and one of the more highly ranked in VACV as a whole. The relatively high immunogenicity of this epitope led to a reevaluation of the total CD8(+) T cell response to VACV. By the use of two methods, the true size of the response was found to be around double previous estimates and at its peak is on the order of 60% of all CD8(+) T cells. We speculate that more CD8(+) T cell epitopes remain to be mapped for VACV and that underestimation of responses is unlikely to be unique to VACV, so there would be merit in revisiting this issue for other viruses.

Age-dependent susceptibility to a viral disease due to decreased natural killer cell numbers and trafficking

Although it is well known that aged hosts are generally more susceptible to viral diseases than the young, specific dysfunctions of the immune system directly responsible for this increased susceptibility have yet to be identified. We show that mice genetically resistant to mousepox (the mouse parallel of human smallpox) lose resistance at mid-age. Surprisingly, this loss of resistance is not a result of intrinsically defective T cell responses. Instead, the primary reason for the loss of resistance results from a decreased number of total and mature natural killer (NK) cells in the blood and an intrinsic impairment in their ability to migrate to the lymph node draining the site of infection, which is essential to curb systemic virus spread. Hence, our work links the age-dependent increase in susceptibility to a viral disease to a specific defect of NK cells, opening the possibility of exploring treatments to improve NK cell function in the aged with the goal of enhancing their resistance to viral diseases.