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The marked proliferation of activated CD8 + T cells is pathognomonic of EBV-associated infectious mononucleosis (IM), common in young adults. Since the diversity and size of the memory CD8 + T cell population increase with age, we questioned whether IM was mediated by the reactivation of memory CD8 + T cells specific to previously encountered pathogens but cross-reactive with EBV. Of 8 HLA-A2 + IM patients, 5 had activated T cells specific to another common virus, as evidenced by a significantly higher number of peripheral blood influenza A virus M1 58-66 -specific T cells compared with healthy immune donors. Two patients with an augmented M1 response had tetramer-defined cross-reactive cells recognizing influenza M1 and EBV-BMLF1 280-288 , which accounted for up to one-third of their BMLF1-specific population and likely contributed to a skewed M1-specific T cell receptor repertoire. These epitopes, with only 33% sequence similarity, mediated differential effects on the function of the cross-reactive T cells, which may contribute to alterations in disease outcome. EBV could potentially encode an extensive pool of T cell epitopes that activate other cross-reactive memory T cells. Our results support the concept that cross-reactive memory CD8 + T cells activated by EBV contribute to the characteristic lymphoproliferation of IM.

The main functions of memory T cells are to provide protection upon re-exposure to a pathogen and to prevent the re-emergence of low-grade persistent pathogens. Memory T cells achieve these functions through their high frequency and elevated activation state, which lead to rapid responses upon antigenic challenge. The significance and characteristics of memory CD8+ T cells in viral infections have been studied extensively. In many of these studies of T-cell memory, experimental viral immunologists go to great lengths to assure that their animal colonies are free of endogenous pathogens in order to design reproducible experiments. These experimental results are then thought to provide the basis for our understanding of human immune responses to viruses. Although these findings can be enlightening, humans are not immunologically naïve, and they often have memory T-cell populations that can cross-react with and respond to a new infectious agent or cross-react with allo-antigens and influence the success of tissue transplantation. These cross-reactive T cells can become activated and modulate the immune response and outcome of subsequent heterologous infections, a phenomenon we have termed heterologous immunity. These large memory populations are also accommodated into a finite immune system, requiring that the host makes room for each new population of memory cell. It appears that memory cells are part of a continually evolving interactive network, where with each new infection there is an alteration in the frequencies, distributions, and activities of memory cells generated in response to previous infections and allo-antigens.

Here we demonstrate complex networks of CD8 T-cell cross-reactivities between influenza A virus (IAV) and Epstein- Barr virus (EBV) in humans and between lymphocytic choriomeningitis virus (LCMV) and vaccinia virus (VV) in mice. We also show directly that cross-reactive T-cells mediate protective heterologous immunity in mice. Subsets of T-cell populations reactive with one epitope cross-reacted with either of several other epitopes encoded by the same or the heterologous virus. Human T-cells specific to EBV-encoded BMLF1280-288 could be cross-reactive with two IAV or two other EBV epitopes. Mouse T-cells specific to the VV-encoded a11r198-205 could be cross-reactive with three different LCMV, one Pichinde virus, or one other VV epitope. Patterns of cross-reactivity differed among individuals, reflecting the private specificities of the host’s immune repertoire, and divergence in the abilities of T-cell populations to mediate protective immunity. Defining such cross-reactive networks between commonly encountered human pathogens may facilitate the design of vaccines.

The nature of CD8+ T cell memory is still incompletely understood. We have previously reported that the response to an HLA-A2-restricted influenza-derived peptide results in a complex T cell repertoire. In this study we extend this analysis and describe the repertoire with more rigor. In one individual we defined 141 distinct T cell clonotypes on the basis of the unique DNA sequence of the third complementarity-determining region of the TCR β-chain. The frequency distribution of the clonotypes is not what is expected of a normal distribution but is characterized by a large low-frequency tail. The existence of a complex population indicates a mechanism for maintaining a large number of Ag-specific clonotypes at a low frequency in the memory pool. Ranking the clonotypes allowed us to describe the population in terms of a power law-like distribution with a parameter of decay of ∼1.6. If the repertoire is divided into subsets, such as clonotypes that use BJ2.7 or those whose third complementarity-determining region encodes the amino acid sequence IRSS, the clonotype frequencies could also be described by a power law-like distribution. This indicates a self similarity to the repertoire in which smaller pieces are slightly altered copies of the larger piece. The power law-like description is stable with time and was observed in a second individual. The distribution of clonotypes in the repertoire could be mapped onto a polygonal spiral using a recursive algorithm. Self similarity, power laws, and recursive mapping algorithms are associated with fractal systems. Thus, Ag-specific memory CD8 T cell repertoires can be considered as fractal, which could indicate optimized flexibility and robustness.

Alterations in memory CD8 T cell responses may contribute to the high morbidity and mortality caused by seasonal influenza A virus (IAV) infections in older individuals. We questioned whether memory CD8 responses to this nonpersistent virus, to which recurrent exposure with new strains is common, changed over time with increasing age. Here, we show a direct correlation between increasing age and narrowing of the HLA-A2-restricted IAV V␣ and V␤ T cell repertoires specific to M1 residues 58 to 66 (M1 58 -66 ), which simultaneously lead to oligoclonal expansions, including the usage of a single identical VA12-JA29 clonotype in all eight older donors. The V␣ repertoire of older individuals also had longer CDR3 regions with increased usage of G/A runs, whose molecular flexibility may enhance T cell receptor (TCR) promiscuity. Collectively, these results suggest that CD8 memory T cell responses to nonpersistent viruses like IAV in humans are dynamic, and with aging there is a reduced diversity but a preferential retention of T cell repertoires with features of enhanced crossreactivity. IMPORTANCEWith increasing age, the immune system undergoes drastic changes, and older individuals have declined resistance to infections. Vaccinations become less effective, and infection with influenza A virus in older individuals is associated with higher morbidity and mortality. Here, we questioned whether T cell responses directed against the highly conserved HLA-A2-restricted M1 58 -66 peptide of IAV evolves with increasing age. Specifically, we postulated that CD8 T cell repertoires narrow with recurrent exposure and may thus be less efficient in response to new infections with new strains of IAV. Detailed analyses of the VA and VB TCR repertoires simultaneously showed a direct correlation between increasing age and narrowing of the TCR repertoire. Features of the TCRs indicated potentially enhanced cross-reactivity in all older donors. In summary, T cell repertoire analysis in older individuals may be useful as one of the predictors of protection after vaccination. Remarkable progress in medicine combined with public health policy and socioeconomic development has resulted in longer life spans than ever before. It was predicted that in 2050 the proportion of people aged 65 years and older in the United States will more than double to 22% from the 10% in the year 2000 (1). This will present challenges and will have an impact on health care systems, and reversing the negative effects of aging would profit individuals and society. An age-related phenomenon known as "immunosenescence," where both innate and adaptive immune responses become impaired, can result in deleterious clinical endpoints (2). This process is linked to declining resistance to infections and the poor efficacy of vaccines in older individuals (3).Earlier reports have shown that T cell receptor (TCR) repertoire diversity is a key component of protection from infection (4-7). TCR gene sequences present unique markers for tracking T cell diversity. During agi...

Memory T cells cross-reactive with epitopes encoded by related or even unrelated viruses may alter the immune response and pathogenesis of infection by a process known as heterologous immunity. Because a challenge virus epitope may react with only a subset of the T cell repertoire in a cross-reactive epitope-specific memory pool, the vigorous cross-reactive response may be narrowly focused, or oligoclonal. We show here, by examining human T cell cross-reactivity between the HLA-A2-restricted influenza A virus-encoded M158-66 epitope (GILGFVFTL) and the dissimilar Epstein-Barr virus-encoded BMLF1280-288 epitope (GLCTLVAML), that under some conditions heterologous immunity can lead to a significant broadening rather than a narrowing of the T cell receptor repertoire. We suggest that dissimilar cross-reactive epitopes might generate a broad rather than narrow T cell repertoire if there is a lack of dominant high affinity clones, and this hypothesis is supported by computer simulation.