Every year, millions of people across the world become infected by the influenza virus, resulting in an average of between 250,000 and 500,000 fatalities annually, and over $80 billion in costs in the United States alone. Current seasonal influenza vaccines consist of three different, common strains of inactivated influenza virus. These inactivated viruses cannot replicate or cause pathogenic infection, but the viral proteins are recognized by the host immune system and evoke an immune response that leads to generation of specific cells that have greater efficiency in clearing the virus upon second exposure. Current research is thus focused on potential vaccines and treatments that can augment the immune response to favor generation of cells with the best ability to clear influenza viral infection.
At the molecular level, viral flu proteins are recognized by receptors on host immune cells called dendritic cells (DCs) or other antigen-presenting cells, which present small bits of viral protein to T cells, which subsequently become “activated” and await second exposure to the virus. Some T cells with particularly high antigen-specificity (in other words, T cells with receptors that bind with optimal affinity to particular influenza viral antigen) differentiate into memory T cells, which “remember” a specific antigen and are able to produce a faster, more efficient immune response upon subsequent exposure. In this case, the memory T cells effectuate a clearance of the virus through several mechanisms, including production of proteins called cytokines, many of which induce inflammation to help clear viral infection. A specific class of T cells called helper T cells, or CD4 T cells, has been demonstrated to be particularly helpful in the immune response against viral infection because of their particular subset of secreted cytokines. A paper published just this month in The Journal of Immunology by Teijaro and colleagues describes a newly identified class of lung tissue-resident memory CD4 T cells with enhanced ability to provide protection during respiratory viral infection. The authors utilized several distinct mouse models to demonstrate T cell retention in the lung after influenza virus infection and increased protection from morbidity from the virus.
First, Teijara and colleagues infected mice with a sub-lethal dose of the influenza virus and examined the generation of influenza-specific memory CD4 T cells, with two populations in the lung and spleen. As determined by ELISPOT assay, CD4 T cells in the lungs expressed higher quantities of interferon-gamma (IFN-γ), a pro-inflammatory cytokine. The authors sought to determine the significance of tissue location—independent of Ag specificity—in the distinct functions of lung and spleen memory CD4 T cells. To accomplish this, they transferred memory CD4 T cells with the same T cell receptor clonotype (and thus the same Ag specificity) into lymphocyte-deficient mice. Memory CD4 T cells in the lung had fewer IL-2-producing cells and a similar amount of IFN-γ-producing cells relative to the cell population in the spleen, suggesting tissue-specific influence on the function of resident memory T cell populations, independent of Ag specificity.
The authors hypothesized that tissue location of these cells was also critical in influencing their homing-capacity. Lung and spleen memory CD4 T cells were isolated and transferred into unaltered hosts, and their resulting anatomical distribution was analyzed one to three weeks later. Intriguingly, while spleen-derived cells dispersed into multiple tissues, including spleen, liver, lung, and lymph nodes, lung-derived cells distributed almost entirely into the lungs of recipient mice, and all of the lung-derived cells were recovered from the lungs of recipient mice three weeks after transplant. This indicates that lung-resident CD4 T cells are distinct from their splenic counterparts in function and migration, and are able to home to the lungs.
Next, the authors sought to determine if the lung-specific memory CD4 T cells homed to the lungs via recirculation and migration back to the lung, or by initial retention in the lung. To determine this, they utilized a parabiosis model in which two mice were surgically joined. Host mice containing either spleen-derived or lung-derived memory CD4 T cells, respectively, were joined with unaltered partner mice, and T cell populations in multiple tissues were recovered and analyzed. In mouse pairs containing the spleen-derived cells, memory CD4 T cells were detected with considerable frequencies in the spleen, liver, and lungs of both mice. Interestingly, mouse pairs containing the lung-derived CD4 T cells had memory CD4 T cells exclusively in the lungs of the host mice, with no dispersal at all into additional tissues or into the partner mice, even up to three weeks post-transplant. These results demonstrated that the lung memory CD4 T cells have a strong retention in the lung tissue, independent of both Ag specificity and inflammation, and that they are distinct from any previously described lung CD8 T cells, which do re-circulate and migrate to various tissues (1).
As a final examination of the immunological protective capacity of these memory CD4 T cells, Teijara and colleagues infected naïve mice – mice with no prior exposure to the influenza virus –, and mice recipients of spleen- or lung-derived memory CD4 T cells, respectively, with lethal and sub-lethal doses of the influenza PR8 (H1N1) virus, and monitored weight loss, morbidity, and viral load. After sub-lethal infection, naïve and spleen-derived cell recipient mice exhibited severe weight loss of up to 25% by 5 days post-infection, while lung-memory cell recipients only showed mild weight loss of 10% throughout the infection. Interestingly, lung-derived cell recipients were able to more rapidly clear the viral infection than the spleen-derived and naïve mice. Lung-derived memory CD4 T cell recipients were, on average, able to completely clear the viral infection by eight days post-infection. Additionally, these mice were fully protected from lethal infection compared to naive mice (60% morbidity) and spleen-derived memory T cell recipients, which actually experienced accelerated death. This further demonstrates that lung memory CD4 T cells serve as optimal protectors against influenza infection.
Moreover, this study identifies a novel type of lung-specific memory CD4 T cell with increased ability to afford protection against viral lung infection, and indicates that protective capacity of memory T cells is linked to their tissue compartmentalization. This also suggests that perhaps simple quantification of circulating Ag-specific T cells, typically considered indicative of the potency of an immune response, may not reflect the quality of a local memory T cell population response. In light of this study, new research in augmenting current flu vaccines or treatments should consider targeting the promotion and maintenance of these tissue-resident memory CD4 T cell populations, perhaps via upregulation of secreted cytokines that promote memory CD4 T cell growth and development.
Reference: Teijaro, J. R., Turner, D., Pham, Q., Wherry, E. J., Lefrançois, L., & Farber, D. L. (2011). Cutting edge: tissue-retentive lung memory CD4 T cells mediate optimal protection to respiratory virus infection. Journal of immunology (Baltimore, Md. : 1950), 187(11), 5510-4. doi:10.4049/jimmunol.1102243
(1) Masopust, D., V. Vezys, E. J. Usherwood, L. S. Cauley, S. Olson, A. L. Marzo, R. L. Ward, D. L. Woodland, and L. Lefrançois. 2004. Activated primary and memory CD8 T cells migrate to nonlymphoid tissues regardless of site of activation or tissue of origin. J. Immunol. 172: 4875–4882.