Ageing and Toll-like receptor expression by innate immune cells in chronic human schistosomiasis

Abstract

There has been no systematic study of the immune response of individuals aged over 60 years living in Schistosomiasis mansoni-endemic areas, although senescence is reportedly associated with susceptibility to infection and progressive decline in immune function. We have shown previously, in two endemic areas in Minas Gerais, Brazil, that the frequency of individuals aged over 60 years with chronic schistosomiasis is no longer negligible. Moreover, several elderly individuals who have always lived in these endemic areas stay protected from infection. An important question for studies of ageing and disease control in developing countries is which differences in the immunological profile of these negatively tested (non-infected) individuals can account for their resistance to either infection or reinfection. We show, in the present study, that non-infected (negative) elderly individuals develop innate immune mechanisms of protection that replace the age-associated decline in T cell function. Non-infected elderly individuals from endemic areas of schistosome infection present an increase in the frequency of the natural killer (NK) CD56(low) subset of NK cells expressing Toll-like receptors (TLR)-1, -2, -3 and -4 as determined by flow cytometry analysis. In addition, the proportion of dendritic cells expressing TLR-1 is elevated as well as the frequency of monocytes expressing TLR-1 and -4. These results suggest that TLR expression by cells of the innate immune system may be related to the negative status of infection in some elderly individuals who are constantly exposed to S. mansoni. Developing mechanisms of protection from infection may represent a biomarker for healthy ageing in this population.

Fig. 1

Frequency of CD56⁺, CD56^low and CD56^high natural killer (NK) cells in individuals from Travessão, an area in Brazil endemic for schistosomiasis. Nine non-infected and eight infected individuals were divided into two age groups: 15–59 (adults) and 60–77 years (elderly). Statistical analysis was performed as explained in the Subjects and methods section. Each point represents one individual. (a) NK cells were identified in flow cytometry studies by their light-scatter–side-scatter (SSC) and forward-scatter (FSC) characteristics and by the expression of CD56 (gated R1). (b) There is a trend to an increase in the frequency of CD56⁺ cells in non-infected and infected individuals aged over 60 years compared to adult individuals. (c) Two major NK subsets were also analysed: CD56^low (gated R3) and CD56^high (gated R4) cells. (d) The proportion of CD56^low cells increased and (d) the proportion of CD56^high declined in infected elderly individuals compared to individuals in the infected adult group. Infect adult = infected adult and Infect elderly = infected elderly.

Fig. 2

Frequencies of Toll-like receptor (TLR)-1, TLR-2, TLR-3 and TLR-4-expressing CD56⁺ natural killer (NK) cells in individuals from Travessão, an area in Brazil endemic for schistosomiasis. Nine non-infected and eight infected individuals were divided into two age groups: 15–59 (adults) and 60–77 years (elderly). Statistical analysis was performed as explained in the Subjects and methods section. Each point represents one individual. (a) There was a significant increase in the frequency of CD56⁺ TLR-1⁺ in non-infected elderly people compared to the non-infected adult groups. No difference was observed in the frequency of NK cells expressing either TLR-2, TLR-3 or TLR-4 among the groups (b, c, d). Infect adult = infected adult and Infect elderly = infected elderly.

Fig. 3

Frequencies of Toll-like receptor (TLR)-, TLR2-, TLR3- and TLR4-expressing CD56^low natural killer (NK) cells in individuals from Travessão, an area in Brazil endemic for schistosomiasis. Nine non-infected and eight infected individuals were divided into two age groups: 15–59 (adults) and 60–77 years (elderly). Statistical analysis was performed as explained in the Subjects and methods section. Each point represents one individual. There is an increase in the frequency of CD56^low TLR-1⁺ (a), CD56^low TLR-2⁺ (b) and CD56^low TLR-3⁺ (c) cells in non-infected elderly individuals compared to non-infected adults and infected elderly subjects. Frequency of CD56^low cells expressing TLR-4 was different only between healthy elderly and healthy adult individuals (d). Infect adult = infected adult and Infect elderly = infected elderly.

Fig. 4

Frequencies of Toll-like receptor (TLR)-1, TLR-2-, TLR-3- and TLR-4-expressing CD56^high natural killer (NK) cells in individuals from Travessão, an area in Brazil endemic for schistosomiasis. Nine non-infected and eight infected individuals were divided into two age groups: 15–59 (adults) and 60–77 years (elderly). Statistical analysis was performed as explained in the Subjects and methods section. Each point represents one individual. There was an increase in the frequency of TLR-1- and TLR-4-expressing CD56^high cells in infected elderly people compared to the infected adult group (a, d). Frequencies of CD56^high cells expressing either TLR-2 and TLR-3 were not different among groups (b, c). Infect adult = infected adult and Infect elderly = infected elderly.

Fig. 5

Frequency of Toll-like receptor (TLR)-1, TLR-2-, TLR-3- and TLR-4-expressing dendritic cells (DC) in individuals from Travessão, an area in Brazil endemic for schistosomiasis. Nine non-infected and eight infected individuals were divided into two age groups: 15–59 (adults) and 60–77 years (elderly). Statistical analysis was performed as explained in the Subjects and methods section. Each point represents one individual. (a) DC were detected in flow cytometry studies by their characteristic profile at the light scatter and by expression of CD11c. (b) Frequency of CD11c⁺ expressing TLR-1 was increased to almost 100% in elderly non-infected individuals compared to non-infected adult and infected elderly groups. Frequencies of TLR-2-, TLR-3- and TLR-4-expressing DC were not different among groups (c, d, e). Infect adult = infected adult and Infect elderly = infected elderly.

Fig. 6

Frequencies of Toll-like receptor (TLR)-1, TLR-2 and TLR-4-expressing monocytes in individuals from Caju, an endemic area for schistosomiasis in Brazil. Twenty-two non-infected and 16 infected individuals were divided into two age groups: 19–59 (adults) and 68–94 years (elderly). Statistical analysis was performed as explained in the Subjects and methods section. Each point represents one individual. (a) Monocytes were detected in flow cytometry studies by their characteristic profile at the light scatter and by expression of CD14. (b) Frequency of CD14⁺ cells expressing TLR-1 was increased in elderly non-infected individuals compared to the infected elderly group. (c) The frequency of TLR-2-expressing monocytes was not different among groups. (d) The frequency of CD14⁺ cells expressing TLR-4⁺ was increased when compared to both the infected elderly and the non-infected groups. Infect adult = infected adult and Infect elderly = infected elderly.