Early Life Inflammation and the Developing Hematopoietic and Immune Systems: The Cochlea as a Sensitive Indicator of Disruption

Abstract

Emerging evidence indicates that perinatal infection and inflammation can influence the developing immune system and may ultimately affect long-term health and disease outcomes in offspring by perturbing tissue and immune homeostasis. We posit that perinatal inflammation influences immune outcomes in offspring by perturbing (1) the development and function of fetal-derived immune cells that regulate tissue development and homeostasis, and (2) the establishment and function of developing hematopoietic stem cells (HSCs) that continually generate immune cells across the lifespan. To disentangle the complexities of these interlinked systems, we propose the cochlea as an ideal model tissue to investigate how perinatal infection affects immune, tissue, and stem cell development. The cochlea contains complex tissue architecture and a rich immune milieu that is established during early life. A wide range of congenital infections cause cochlea dysfunction and sensorineural hearing loss (SNHL), likely attributable to early life inflammation. Furthermore, we show that both immune cells and bone marrow hematopoietic progenitors can be simultaneously analyzed within neonatal cochlear samples. Future work investigating the pathogenesis of SNHL in the context of congenital infection will therefore provide critical information on how perinatal inflammation drives disease susceptibility in offspring.

Keywords: cochlea; congenital infection; cytomegalovirus; fetal-derived immune cells; hematopoiesis; hematopoietic stem and progenitor cells; inflammation; sensorineural hearing loss.

Figure 1

Early life inflammation as a driver of disease susceptibility. Perinatal inflammation can drive lasting changes to immune function from fetal development onwards by perturbing immune function at the cellular, tissue, and whole organism level. Impaired development of fetal-derived immune cells that influence tissue development and homeostasis can ultimately impair tissue function. Specifically, fetal-derived immune cell compartments can either be expanded or reduced by inflammation, or cells can by hyper- or hypo-activated. Perturbing the establishment and function of developing fetal hematopoietic stem and progenitor cells (HSPCs) by inducing loss or persistence of transient HSPCs, or shaping function and output of developing HSCs may also result in lasting changes to the composition or output of the adult hematopoietic stem cell (HSC) compartment. Ultimately, the effects of perinatal inflammation on both developing HSPCs and fetal-derived immune cells may reshape the trajectory of postnatal immunity and susceptibility to disease by impinging on tissue immunity and/or the output of the hematopoietic system.

Figure 2

Anatomy of the ear and auditory system. (A). Outer, middle, and inner ear shown with labelled anatomic structures that are important for hearing. (B). Illustration of a human cochlea with the cochlear base and apex labelled. Hair cells at the cochlear base interpret high-pitch sounds and hair cells at the apex interpret low-pitch sounds. The cross section of the cochlea shows the location of perilymph and endolymph. Hair cells resonate with incoming sound waves and send impulses to the brain via the spiral ganglion.

Figure 3

Sensorineural hearing loss can be assessed by auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE) testing. (A). Comparison of ABR waveforms from a mouse with normal hearing and a mouse with severe to profound hearing loss. Higher thresholds indicate worse hearing, and lower thresholds indicate better hearing. Detected ABR signals are measured in microvolts (μV) over time in milliseconds (ms), and hearing thresholds are determined as decibels of sound pressure level (dB SPL) based on the presented auditory stimulus. (B). DPOAE results showing peaks from presented stimuli (Frequency 1 [F1] and Frequency 2 [F2] peaks) and the presence of a distortion product (DP peak). The presence of the distortion product indicates that OHCs are responding properly to the noise stimulus with detected signals measured in dB SPL.

Figure 4

The developing cochlea contains a rich immune environment. Flow cytometric analysis of cochlear and temporal bone immune populations of post-natal day 10 mice. Cochlea and temporal bone were dissected as described in the methods section and similar to Jan et al. [128]. (A). Comparison of innate and adaptive immune populations as defined in the methods section as a percentage of CD45+ cells. (B). Total cellularity of innate and adaptive immune populations in the cochlea. Error bars represent the mean ± SEM. N = 5–6 mice per tissue structure.

Figure 5

Temporal bone hematopoietic stem and progenitor cells (HSPCs) are sensitive to perinatal infection with CMV. Flow cytometric analysis of HSPCs (Lin− c-kit+ Sca-1+) of post-natal day 10 mice that were treated with saline or mCMV at post-natal day 3. (A). Long bone (femur and tibia) marrow HSPCs significantly expand in response CMV infection (7dpi). (B). Temporal bone contains fewer HSPCs than tibial bone marrow, but similarly expand in response to CMV infection. Error bars represent the mean ± SEM. N = 7–13 mice per treatment across 2–4 litters. * p < 0.05, *** p < 0.001.