Experimental Human Pneumococcal Colonization in Older Adults Is Feasible and Safe, Not Immunogenic

Abstract

Rationale: Pneumococcal colonization is key to the pathogenesis of invasive disease but is also immunogenic in young adults, protecting against recolonization. Colonization is rarely detected in older adults, despite high rates of pneumococcal disease.Objectives: To establish experimental human pneumococcal colonization in healthy adults aged 50-84 years, to measure the immune response to pneumococcal challenge, and to assess the protective effect of prior colonization against autologous strain rechallenge.Methods: Sixty-four participants were inoculated with Streptococcus pneumoniae (serotype 6B; 80,000 cfu in each nostril). Colonization was determined by bacterial culture of nasal wash, and humoral immune responses were assessed by anticapsular and antiprotein IgG concentrations.Measurements and Main Results: Experimental colonization was established in 39% of participants (25/64) with no adverse events. Colonization occurred in 47% (9/19) of participants aged 50-59 compared with 21% (3/14) in those aged ≥70 years. Previous pneumococcal polysaccharide vaccination did not protect against colonization. Colonization did not confer serotype-specific immune boosting, with a geometric mean titer (95% confidence interval) of 2.7 μg/ml (1.9-3.8) before the challenge versus 3.0 (1.9-4.7) 4 weeks after colonization (P = 0.53). Furthermore, pneumococcal challenge without colonization led to a drop in specific antibody concentrations from 2.8 μg/ml (2.0-3.9) to 2.2 μg/ml (1.6-3.0) after the challenge (P = 0.006). Antiprotein antibody concentrations increased after successful colonization. Rechallenge with the same strain after a median of 8.5 months (interquartile range, 6.7-10.1) led to recolonization in 5/16 (31%).Conclusions: In older adults, experimental pneumococcal colonization is feasible and safe but demonstrates different immunological outcomes compared with younger adults in previous studies.

Keywords: Streptococcus pneumoniae;; elderly; human challenge models; immunity; vaccination.

Figure 1.

Timeline for the study, including the optional rechallenge (for participants who developed colonization during the primary study) up to 1 year later.

Figure 2.

Microbiological status of participants before and after inoculation. Three participants were naturally colonized with non-6B serotypes at baseline, two of whom subsequently developed experimental colonization after inoculation (see online supplement for details); 23 previously uncolonized participants developed experimental colonization, and colonization with nonexperimental strains (serogroups 15, 20, and non–vaccine-type, Statens Serum Institut group G). At screening before rechallenge up to 1 year later, one volunteer was naturally colonized (with serogroup 15), and another was still colonized with the experimental strain; the latter was treated with amoxicillin and excluded from the remainder of the study. Five of the remaining 16 participants developed experimental colonization after rechallenge.

Figure 3.

Experimental colonization rates in older adults (defined by positive pneumococcal culture in nasal wash at any time point after inoculation) compared with a young adult cohort (from similar studies conducted during the same time period) and broken down by age decile within the older cohort. Numbers denote (number colonized)/(total number in that age category). Error bars represent 95% confidence intervals. There were no statistically significant differences between the older age category or subcategories and the younger volunteers.

Figure 4.

(A–F) Anti-6B capsular polysaccharide IgG concentrations at baseline and Day 29 after inoculation in the full cohort (A and B), participants who had never received pneumococcal polysaccharide vaccine (C and D) and participants who had received pneumococcal polysaccharide vaccine (E and F). Each symbol represents a single participant. The lines and error bars in A, C, and E represent geometric means (95% confidence intervals); the lines in B, D, and F connect the baseline and Day 29 values for each participant.

Figure 5.

Changes in antibody concentrations against 27 different pneumococcal proteins after nasopharyngeal pneumococcal challenge, measured by multiplex electrochemiluminescence (Meso Scale Discovery), in (A) noncolonized and (B) colonized participants. Antibodies are expressed as fold change between prechallenge baseline and 29 days after inoculation with Streptococcus pneumoniae. Error bars represent mean (SEM). Significant differences in fold change between carriers and noncarriers were analyzed using multivariate regression: *P < 0.05 and **P < 0.001.

Figure 6.

Serum anti-6B capsular antibodies before and after the primary and rechallenge phases of the study. Only participants who were colonized after the primary challenge and returned for rechallenge are shown. They are subdivided into those who did and did not become colonized after rechallenge. Each circle represents an individual participant.