Our study demonstrates that the ability of elderly individuals to respond to a trivalent influenza vaccine is low, with only 17% of vaccine recipients mounting an increase in titer of antibody to all three vaccine components. Among these aged individuals, 46% failed to generate an immune response to any of the hemagglutinins of a trivalent influenza vaccine. Within this age group (65 to 98 years), age negatively correlated with the induction of anti-influenza virus antibodies, providing evidence for a critical role of immunosenescence in compromising the effectiveness of vaccination. Data presented here suggest that measuring the frequency of CD8+ CD28null T cells, a biological marker that can be easily assessed, provides useful information about the immunocompetence of aged individuals, particularly as it relates to their ability to generate vaccine-induced humoral responses. Confirmatory studies are needed to determine whether frequencies of circulating CD8+ CD28null T cells can be developed into a reliable biological marker to identify individuals at risk for poor anti-influenza virus vaccine response. In addition, our observations could be helpful in designing novel mechanistic approaches for improving immunocompetence in the elderly.
Postvaccination hemagglutination inhibition antibody titers are generally accepted as inversely correlating with the susceptibility to infection, with a titer of ≥40 being considered protective (36
). In this study, we wanted to assess the ability to respond to vaccination with a humoral immune response and therefore used the titer increase as the outcome variable. With this criterion, a large proportion of our study population did not respond to the vaccination, although the population excluded individuals with major comorbidity. Immunogenicity may vary between influenza virus vaccine strains, and indeed A/Texas/36/91 has been described as poorly immunogenic (16
). Therefore, it cannot be excluded that elderly individuals may have a better response to other vaccine strains. However, it was striking that nearly half of the population did not mount a humoral response to any of the three vaccine strains.
Our findings on vaccine responses of the elderly are consistent with previous studies. In a community-based cohort study by Nichol et al. (33
), vaccination was shown to prevent 48 to 57% of all hospitalizations for pneumonia and influenza. Similar results have been reported from several case-control observational studies. Thus, there is no question that vaccination with inactivated influenza virus is effective in a large subset of elderly individuals and is, therefore, cost-effective. It is also evident that the vaccine immunogenicity is far below what is desired to prevent the disease in this risk group. The frequency of CD8+
T cells appears to be a good biological marker to identify individuals at risk, even after adjustments for other risk factors, such as age. It is obvious that this vaccine nonresponder population does not benefit from the vaccination.
In addition to the immunocompetence of the host, immune responses to vaccination are influenced by several other variables, in particular by preexisting immunity. Prevaccination titers varied substantially in our study population; the majority of individuals already had titers of 40 and more. Vaccinees with higher baseline titers have a reduced chance to respond to the vaccination and, therefore, do not develop immunity to the new antigenic epitopes unique for the vaccine strain (16
). Smith et al. (41
) recently analyzed this phenomenon using historical data from repeat vaccinations, including the relatedness of the vaccine antigens employed in subsequent years and the vaccination efficacy (3
). Using computer simulation, they demonstrated that the similarity of the vaccine strain to previous strains inversely correlated with the efficacy of repeat vaccinations. Our findings are consistent with this model. For all three antigens, the preexisting antibody titer had a negative impact on the vaccine response. Interestingly, there was no difference in this respect between A/Texas, which had been used for vaccinations in previous years and had the highest prevaccination titers, and the two other vaccine strains. Most importantly, multivariate analysis showed that CD8+
-T-cell frequencies remained predictive of a poor vaccine response even after adjustment for baseline titers.
The correlation between CD8+
T cells and the defective antibody responses to vaccination may also give mechanistic insights. Few studies have tried to develop immune status indices that are predictive of immunosenescence. The tests either are of limited value or are complicated, e.g., skin testing for delayed-type hypersensitivity or in vitro T-cell assays (10
). The three tests selected here represent phenotypic markers of T-cell senescence that can be easily assessed by flow cytometry. CD45RO is a cell surface molecule that is expressed on T cells that have recognized antigen in the past (38
). Therefore, it has been assumed that repeated exposure to antigens during a lifetime would result in an increase in the CD45RO+
population and lead to an exhaustion of the CD45ROnull
(naive) population. The CD45RO+
(memory) population is at least 100-fold less diverse than the naive T-cell subset (2
). Therefore, an expansion of this subset at the expense of the naive population would compromise the ability to generate an immune response to new antigens, which is required to recognize the antigenic drifts of the influenza virus hemagglutinins. Indeed, animal data support the notion that CD45 isoforms may be useful markers of immunosenescence (30
). Our studies have clearly shown that this is not the case.
The CD28 molecule transmits signals that are pivotal for T-cell activation and T-cell survival (23
). In the absence of CD28 triggering, T cells are rendered anergic, have a limited ability to proliferate, and are prone to apoptosis. With continuous replication, T cells tend to lose two DNA-binding proteins that are necessary for the transcription of the CD28 gene (43
). The loss of these two binding proteins can be more easily induced in CD8 than in CD4 T cells. The emergence of CD8+
T cells is, therefore, normal with aging and chronic infections. The frequency of CD8+
T cells followed a Gaussian distribution, yet with a very wide range, indicating marked diversity in the elderly population. CD4+
T cells are less frequent and emerge in only a subset of healthy individuals (26
). The finding that the frequency of CD8+
T cells is predictive of a poor antibody response to the influenza vaccine may indicate that the loss of CD28 not only is a marker of immunosenescence but also is directly responsible. CD28-mediated costimulation has been shown to be crucial for the formation of germinal centers, where B-cell responses mature and antigen-specific high-affinity antibodies are generated. Mice treated with anti-CD28 antibodies and CD28 knockout mice have defective germinal centers (11
). In response to immunization, antibodies are not hypermutated and do not undergo affinity maturation, presumably because of defective T-helper-cell function. T-helper-cell activity has generally been attributed to CD4 T cells, which express cell surface molecules such as CD40 ligand and secrete cytokines that enable them to provide B-cell help (44
). Therefore, it was surprising to find that a functional molecule on CD8 T cells correlated with humoral nonresponsiveness. CD8 T cells have been associated with antiviral effector activities rather than with auxiliary regulatory function (28
). However, recent observations in patients with rheumatoid arthritis support a role of a distinct subset of CD8 T cells different from the usual cytotoxic CD8 T cell in germinal-center formation (19
). These studies await confirmation for normal humoral responses. Strategies to restore CD28 expression may, therefore, be useful for improving vaccine responsiveness in the elderly. Preliminary results from our laboratory have indicated that the loss and reexpression of CD28 are regulated by cytokines.
In summary, we have identified a biological marker that can be easily determined and correlates with a defective antibody response to inactivated influenza virus vaccine in the elderly. Individuals with high frequencies of CD8+ CD28null T cells have an increased likelihood of an insufficient antibody response to the vaccine and may be at risk for infection despite being vaccinated. These individuals may be candidates for prophylactic treatment with neuraminidase inhibitors. Restoring CD28 expression through the use of the appropriate cytokines may be an elegant way to overcome the low immunogenicity of the inactivated vaccine in a subset of the elderly.