Differences in Proinflammatory Cytokines and Monocyte Subtypes in Older as Compared With Younger Kidney Transplant Recipients

Abstract

Background: The number of elderly patients with end-stage kidney disease requiring kidney transplantation continues to grow. Evaluation of healthy older adults has revealed proinflammatory changes in the immune system, which are posited to contribute to age-associated illnesses via "inflamm-aging." Immunologic dysfunction is also associated with impaired control of infections. Whether these immunologic changes are found in older kidney transplant recipients is not currently known, but may have important implications for risk for adverse clinical outcomes.

Methods: Three months after transplant, innate immune phenotype was evaluated by flow cytometry from 60 kidney transplant recipients (22 older [≥60 years] and 38 younger [<60 years old]). Multiplex cytokine testing was used to evaluate plasma cytokine levels. Younger patients were matched to older patients based on transplant type and induction immune suppression.

Results: Older kidney transplant recipients demonstrated decreased frequency of intermediate monocytes (CD14++CD16+) compared with younger patients (1.2% vs 3.3%, P = 0.007), and a trend toward increased frequency of proinflammatory classical monocytes (CD14++CD16-) (94.5% vs 92.1%) (P = 0.065). Increased levels of interferon-gamma (IFN-γ) were seen in older patients.

Conclusions: In this pilot study of kidney transplant recipients, we identified differences in the innate immune system in older as compared with younger patients, including increased levels of IFN-γ. This suggests that age-associated nonspecific inflammation persists despite immune suppression. The ability to apply noninvasive testing to transplant recipients will provide tools for patient risk stratification and individualization of immune suppression regimens to improve outcomes after transplantation.

FIGURE 1

Frequency of monocyte subtypes by patient age (younger vs older). PBMC were analyzed for classical (CD14++/CD16−), intermediate (CD14++/CD16+), and nonclassical (CD14+/CD16++) monocytes, expressed as a percentage of the total number of monocytes. Each dot corresponds to a sample; bars indicate median. **P < 0.01 by nonparametric test.

FIGURE 2

Frequency of monocyte subtypes expressing TLR4. A, Frequency of CD14+ monocytes expressing TLR4 (CD284) by patient age (younger vs older). B, Frequency of monocyte subtypes (classical, intermediate, and nonclassical) expressing TLR4 (CD284) by patient age (younger vs older). C, Frequency of CD14+ monocytes (right panel) expressing TLR4 (CD284) by donor type (living (LD) vs deceased donor (DD)). D, Frequency of monocyte subtypes expressing TLR4 (CD284) by donor type. Each dot corresponds to a sample; bars indicate median. *P < 0.05 by nonparametric test.

FIGURE 3

Analysis of cytokine expression by patient age. A, Heatmap by complete linkage hierarchical clustering of log-transformed concentration of cytokines in the plasma of kidney transplant recipients. Yellow indicates higher expression levels, and red indicates lower expression levels. Older patients (≥ age 60 years) indicated by blue blocks, and younger patients by red blocks, on the right hand side of the figure. B, Box and whiskers plot of IFN-gamma expression by older (black squares) and younger (grey circles) patients. Bar shows median and whiskers show interquartile range. *P < 0.05 by nonparametric test and **P < 0.01.