Identification of individuals based on short tandem repeat profiling is limited when no DNA profiles or references are available for comparison. Prediction of phenotypic traits such as eye, hair and skin color or ancestry could provide clues to narrow the range of investigation. Age is proving to be a promising factor in the forensic science field and can be estimated through various methods based on morphological and genetic changes [1, 2]. Among them, age estimation by signal joint T-cell receptor excision circle (sjTREC) quantification has demonstrated considerable prediction accuracy and can be performed using blood or blood stains found at crime scenes [3, 4]. Although several lymphoid organs related to lymphopoiesis and T-cell circulation, such as the thymus and spleen, could also be used for age estimation, sjTRECs levels in these tissues have not yet been measured for this purpose. Thus, this method could be extended to assist with solving unusual murder cases that lack blood remains, such as those involving decapitation or where only a torso remains, because it utilizes other (lymphocyte-containing) body parts as an alternate template for estimating age.

The thymus is a primary lymphoid organ responsible for T-lymphocyte development, including the generation of diverse T-cell receptors. Degeneration of the thymus, called thymic involution, is a process in which the perivascular spaces are gradually replaced with adipose and fibrous tissues, and it is known to progress with age [5]. sjTRECs found in the periphery, known as recent thymic emigrants, are indirect indicators of thymic output, which reflects thymic function, and has also been found to be correlated with age [6]. Therefore, it follows that thymic tissue could be used to estimate age based on sjTREC measurements. Other secondary lymphoid tissues, such as the spleen, could also be used in this way. In the current study, the application of lymphoid organ samples for age estimation using sjTREC levels was investigated for use in forensic science and was applied to samples from real torso murder cases.

It is known that the level of sjTRECs in blood shows a negative correlation with age. The current study investigated whether such a correlation also exists for other human lymphoid organs, namely thymus and spleen. Previous studies using a regression model showed that sjTREC levels were higher in the thymus than in blood or spleen samples [8], and that these levels were maintained in thymocytes until the subjects reached over 60 years of age [8]. This suggests that thymopoiesis may still be ongoing in adults [9], but further studies, with a larger sample size and wider age distribution, would be necessary to confirm this. In particular, this study lacked data from subjects aged in their twenties and below.

When this model was applied to two thymus samples from the torsos of dismembered bodies from real forensic cases, the victims' ages were predicted to be close to their actual age. Considering the fact that the reported standard error of estimation for human blood samples is known to be ±8, the predicted ages from the above two cases were deemed significant. However, further study will be needed to reduce the estimation error and improve the accuracy of the predicted ages. Although thymic involution is known to occur uniformly throughout the organ, it would still be worth investigating whether there are regional differences within the thymic tissues [5], given the fact that on-site sample collection may be limited, depending on the condition of the crime scene and the availability of evidence.

In conclusion, the results from this study demonstrate that measuring sjTREC levels in human thymus and spleen may be a valuable tool for age estimation in forensic applications, particularly in challenging cases, but also as a supplementary tool to existing methods.