In this study, we analyzed serum samples from adult immunocompetent individuals living in The Gambia, South Africa, and the United States in order to assess the prevalence of NAb to Ad5 and Ad35. Using a neutralization assay, we demonstrated that NAb to Ad5 was found both at low and high titers in all populations. In contrast, only a low prevalence of NAb to Ad35 was seen on both continents, and it was found mostly at low titers.
Our findings are similar to other reports. In The Netherlands, Goossens et al. reported that 74% of the studied population exhibited Ad5-NAb and only 4% showed Ad35-NAb, also at low titers (13
). Several studies were performed in the United States. Schulick et al. found a 57% prevalence of Ad5-NAb in an adult U.S. population (29
). In another study, 7 (33.3%) out of 21 mesothelioma patients had NAb to Ad5 (26
). Chen et al. found 46% prevalence of Ad5-NAb in prostate cancer patients and 60% prevalence in the normal healthy population (3
). Thus, one can conclude that between 33 and 60% of the U.S. population have NAb to Ad5 as a result of natural infection.
While a high prevalence of NAb to group C adenoviruses (Ad2 and Ad5 included) was demonstrable in sera of 75 infants, none had NAb to Ad35 (6
). In addition, a low prevalence of NAb to Ad35 was reported in previous assays of pooled gamma globulin (8
). To our knowledge, there is no previously reported adenoviral NAb study in African populations.
Exposure to adenovirus, especially the major viral capsid proteins, usually results in the generation of immunoglobulin G and immunoglobulin A NAb, which attenuate cellular infection upon a second exposure (20
). In the case of Ad5, such circulating NAb have a significant impact on the efficacy and toxicity of systemically administered adenovirus (3
). The use of the Ad5 vector has resulted in fatal toxicity (24
) that was attributed to a high dose of vector (25
). In addition, transgene transfer failure because of preexisting NAb was reported in rheumatoid arthritis patients treated with Ad5 (13
). In another study, a lung cancer patient with high a Ad5-NAb titer experienced lower gene transfer (10
Because of the limitations of currently used adenoviral vectors (Ad2 and Ad5) and the need to improve transgene expression, a search is required for alternative serotypes that are less prevalent in the human population. Ad35 appears to possess qualities of the ideal adenoviral vector. It has a high binding affinity to human epithelial cells (25
). Recently, it was reported to transduce dendritic cells, smooth muscle cells, synoviocytes (33
), and most human cells (30
) more efficiently than Ad5. In addition, Ad35 infection is uncommon in immunocompetent individuals (8
). Our data and that of others (13
) demonstrate the low prevalence of Ad35-specific NAb in immunocompetent individuals from several continents, suggesting the low incidence of human infection caused by this adenovirus serotype worldwide. Given the low or absent titer of preexisting NAb in the general population, minimal doses of an Ad35 vector may be used for gene therapy and vaccine applications, thereby reducing toxicity (25
) without compromising efficacy. In addition, our investigators have recently generated a replication-incompetent Ad35-based vector with E1/E3 deleted in our facility (11
). By using this vector, the desirable qualities of Ad35 will ensure successful delivery and expression of the transgenes without compromising the safety of the recipients. It may be possible, therefore, to achieve the elusive therapeutic goals with Ad35 vectors.
There are some important limitations of our study. The sample size, which was somewhat limited because of the difficulty involved in obtaining serum samples from the studied populations, will need to be increased. Secondly, we studied only populations in Africa and North America. Although similar results were obtained in a European study, it is premature to make global generalizations. It will be interesting to evaluate similar data from Australia, South America, and Asia.
Continued investigation will be important in considering adenoviral vectors as preventive vaccine platforms for HIV and other infectious diseases because of the global prevalence of adenoviral NAb. Based on our findings and those of others, the use of Ad5 as a vector in the preparation of HIV or other vaccines may require caution in African, North American, and European populations. The results of our study demonstrate the importance of screening target populations for adenoviral NAb to specific serotypes before embarking on large-scale adenoviral vector gene therapy or vaccine programs.
This study focused on the determination of preexisting NAb to the candidate vector Ad35 and the commonly used vector Ad5 in three target populations. Given the low prevalence of anti-Ad35 NAb, use of the vector may be suitable for circumventing the humoral response to primary immunization in gene therapy or vaccine applications. Use of the same vector for a second immunization or boost may not be of any therapeutic benefit because of the potential generation of NAb (29
). Based on previous studies (20
), using another vector (serotype) with low preexisting NAb in the population is preferable. Identification of other serotypes with advantages such as those of Ad35 merits further exploration.
Our study did not address the T-cell response to virally infected cells. Cross-reactivity among different adenoviral serotypes arising from recognition by the cellular immune system of adenoviral epitopes expressed by infected cells has been reported (23
). In addition, Flomemberg et al. demonstrated T-cell proliferation in response to Ad35 in individuals without serologic evidence of Ad35 infection (7
In conclusion, our findings provide a broader picture of the prevalence of NAb to the commonly used Ad5 vector and propose a novel candidate vector based on serotype Ad35. Our discovery of low Ad35-specific NAb in the studied populations supports the feasibility of using Ad35 in future gene therapy and vaccine programs.